fix: race condition in private key creation

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• this mitigates a potential vulnerability where a malicious actor could intercept the private key before proper permissions were assigned to the file,
• Removed the needless compilation conditionals - Forest doesn't work outside of Unix and it won't happen anytime soon. If we want to support other platforms, there are way more things to do (and test).

After the change, the permissions are still 0600

drwxr-xr-x. 1 rumcajs rumcajs  70 Aug 22 11:40 ./
drwx------. 1 rumcajs rumcajs 538 Aug 22 11:40 ../
drwxr-xr-x. 1 rumcajs rumcajs  12 Aug 22 11:40 calibnet/
-rw-------. 1 rumcajs rumcajs 161 Aug 22 11:40 keystore
-rw-------. 1 rumcajs rumcajs 116 Aug 22 11:40 keystore.json
drwxr-xr-x. 1 rumcajs rumcajs  14 Aug 22 11:40 libp2p/

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.