For apisupport.ant, don't try to use the nbjdk.bootclasspath ant property, but rather take nbjdk.home, and take the JDK's bootstrap classpath.

[lahodaj]

If I open in a recent build a current source for e.g. <netbeans>/platform/openide.util, I'll get notices like:

Even though the source level of /home/jlahoda/src/nb/netbeans/platform/openide.util.lookup/src:/home/jlahoda/src/nb/netbeans/platform/openide.util.lookup/build/classes-generated is set to: 17, java.lang.AssertionError cannot be found on the bootclasspath: /usr/lib/jvm/java-21-openjdk-amd64/jre/lib/rt.jar

in the log. Looking at the boot class path, that's not really surprising:

/usr/lib/jvm/java-21-openjdk-amd64/jre/lib/rt.jar

there is obviously no rt.jar in the JDK 21 installation, so the search for j.l.AssertionError fails.

This PR tries to fix the apisupport projects boot and module boot path, based on the nbjdk.home property. It will create a (transient) Java platform if there's no Java platform with the given install directory.

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