Native Image Agent Uses JAVA_HOME Instead of GRAALVM_HOME

[dnestoro]

Is your feature request related to a problem? Please describe.

When running gradle -Pagent clean test agent picks JAVA_HOME even though GRAALVM_HOME is set. The problem is that we can set JAVA_HOME to some earlier version (lets say JDK17) and GRAALVM_HOME to the later one (e.g. JDK21), and therefor agent will generate metadata with one JDK version while the Native Image will be created with the different one (example where different metadata can be generated with different Java versions).

Describe the solution you'd like
Make native-image-agent use GRAALVM_HOME as default if possible. Only if GRAALVM_HOME is not configured, use JAVA_HOME.

Describe alternatives you've considered
At least mention in the documentation that agent uses JAVA_HOME