Skip setting JAVA_HOME when using system JDK

[fwcd]

Currently JAVA_HOME is set to the nonexistent path ~/.jenv/versions/system when using the system JDK, which causes a range of strange inconsistencies. For example, the JAVA_HOME will diverge from System.getProperty("java.home"), e.g. like this when using a Homebrew-installed OpenJDK 17 installation:

System.getenv("JAVA_HOME")      => "~/.jenv/versions/system"
System.getProperty("java.home") => "/opt/homebrew/Cellar/openjdk/17.0.2/libexec/openjdk.jdk/Contents/Home"

Additionally, invoking tools like Gradle that rely on JAVA_HOME from the Java application will fail since JAVA_HOME generally takes precedence over other methods of finding a JDK and in this case points to an invalid path.

This PR fixes this by simply not setting JAVA_HOME whenever the system JDK is used.

[bkmeneguello]

Also, the export plugin must be updated to avoid exporting the JAVA_HOME when system is used.

[andrewflbarnes]

Fixes #422 .

Thanks for the observation @bkmeneguello - we can do this in a separate PR given lack of activity in this one

[fwcd]

Ah thanks, sorry for the inactivity!

[andrewflbarnes]

No worries - should have tagged you to check!

Thanks for the contribution @fwcd 😄