Optionally run Gradle build + ITs using different JDK

[snazy]

When the environment variable GRADLE_JAVA_HOME is present, it will be used as the JAVA_HOME when building devtools/gradle/ using Gradle via Maven, Gradle devtools integration tests respect the environment variable as well.

[quarkus-bot]

Failing Jobs - Building ad127ed

Status	Name	Step	Failures	Logs	Raw logs	Build scan
✖	Gradle Tests - JDK 11	Build	Failures	Logs	Raw logs
✔️	Gradle Tests - JDK 11 Windows

Full information is available in the Build summary check run.

Failures

⚙️ Gradle Tests - JDK 11 #

- Failing: integration-tests/gradle 

📦 integration-tests/gradle

✖ io.quarkus.gradle.builder.QuarkusModelBuilderTest.shouldLoadMultiModuleTestModel line 66 - More details - Source on GitHub

org.gradle.tooling.BuildException: Could not run build action using connection to Gradle distribution 'https://services.gradle.org/distributions/gradle-8.3-bin.zip'.
	at org.gradle.tooling.internal.consumer.ExceptionTransformer.transform(ExceptionTransformer.java:51)
	at org.gradle.tooling.internal.consumer.ExceptionTransformer.transform(ExceptionTransformer.java:29)

✖ io.quarkus.gradle.builder.QuarkusModelBuilderTest.shouldLoadMultiModuleDevModel line 100 - More details - Source on GitHub

org.gradle.tooling.BuildException: Could not run build action using connection to Gradle distribution 'https://services.gradle.org/distributions/gradle-8.3-bin.zip'.
	at org.gradle.tooling.internal.consumer.ExceptionTransformer.transform(ExceptionTransformer.java:51)
	at org.gradle.tooling.internal.consumer.ExceptionTransformer.transform(ExceptionTransformer.java:29)

[gsmet]

I will get this in now.
I have a followup PR for trying to consume this in CI (but it will probably require several iterations).

[gsmet]

Thanks!