Enforce JAVA_HOME for ITs

[pzygielo]

This envvar is explicitly used in ITs, for example

exec-maven-plugin/src/it/projects/jigsaw/pom.xml

Line 29 in f75c252

<executable>${JAVA_HOME}/bin/java</executable>

This causes somehow unfriendly failure delayed up to ITs.
Thus I propose to replace

[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.0.1-SNAPSHOT:exec (default-cli) on project mexec-100: Command execution failed. Cannot run program "${JAVA_HOME}/bin/java"

(that one has to check in target/its/mexec-100/build.log) with earlier build failure with

[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireEnvironmentVariable failed with message:
Environment variable "JAVA_HOME" is required for this build.