Make logger instances vars in linux

[Siedlerchr]

Fixes #9908

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

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[HoussemNasri]

Runs flawlessly on OpenSUSE Leap 15, however, I got this exception, don't know if it's relevant.

2023-05-16 19:33:29 [JavaFX Application Thread] org.jabref.preferences.JabRefPreferences.getUser()
ERROR: Hostname not found. Please go to https://docs.jabref.org/ to find possible problem resolution: java.net.UnknownHostException: localhost.localdomain: localhost.localdomain: Name or service not known
	at java.base/java.net.InetAddress.getLocalHost(InetAddress.java:1781)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.getUser(JabRefPreferences.java:2023)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.getInternalPreferences(JabRefPreferences.java:1995)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.getFilePreferences(JabRefPreferences.java:2116)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.getLayoutFormatterPreferences(JabRefPreferences.java:1124)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.lambda$getPreviewLayouts$118(JabRefPreferences.java:2353)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.getPreviewLayouts(JabRefPreferences.java:2357)
	at org.jabref@100.0.0/org.jabref.preferences.JabRefPreferences.getPreviewPreferences(JabRefPreferences.java:2313)
Hostname not found. Please go to https://docs.jabref.org/ to find possible problem resolution: java.net.UnknownHostException: localhost.localdomain: localhost.localdomain: Name or service not known

	at org.jabref@100.0.0/org.jabref.gui.JabRefFrame.createMenu(JabRefFrame.java:814)
	at org.jabref@100.0.0/org.jabref.gui.JabRefFrame.initLayout(JabRefFrame.java:502)
	at org.jabref@100.0.0/org.jabref.gui.JabRefFrame.init(JabRefFrame.java:648)
	at org.jabref@100.0.0/org.jabref.gui.JabRefGUI.openWindow(JabRefGUI.java:79)
	at org.jabref@100.0.0/org.jabref.gui.JabRefGUI.<init>(JabRefGUI.java:59)
	at org.jabref@100.0.0/org.jabref.gui.MainApplication.start(MainApplication.java:31)
	at javafx.graphics@20/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:839)
	at javafx.graphics@20/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:483)
	at javafx.graphics@20/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:456)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
	at javafx.graphics@20/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:455)
	at javafx.graphics@20/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
	at javafx.graphics@20/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
	at javafx.graphics@20/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:316)
	at java.base/java.lang.Thread.run(Thread.java:1589)
Caused by: java.net.UnknownHostException: localhost.localdomain: Name or service not known
	at java.base/java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
	at java.base/java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:52)
	at java.base/java.net.InetAddress$PlatformResolver.lookupByName(InetAddress.java:1059)
	at java.base/java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1673)
	at java.base/java.net.InetAddress$NameServiceAddresses.get(InetAddress.java:1003)
	at java.base/java.net.InetAddress.getAllByName0(InetAddress.java:1663)
	at java.base/java.net.InetAddress.getLocalHost(InetAddress.java:1776)
	... 29 more
Caused by: java.net.UnknownHostException: localhost.localdomain: Name or service not known
` ` ` 

[HoussemNasri]

LGTM!

[Siedlerchr]

Thanks for testing. The other exceptions comes from
userName = get(DEFAULT_OWNER) + '-' + InetAddress.getLocalHost().getHostName();

and indeed it seems to be not that an easy option to get the hostname.
One could add a check InetAddress.getLocalHost().isLoopbackAddress() or in case of the exception try again with
and then do a fallback to call the hostname command
https://stackoverflow.com/questions/7348711/recommended-way-to-get-hostname-in-java