jruby: Fix JAVA_HOME to support proper jmod support

This is investigation I've done in support of
https://github.com/jruby/jruby/issues/6608 where I noticed some funky
issues with JRuby and module loading.

Looks like that JRuby expects JAVA_HOME to have a directory called
`jmod`, which is consistent with the Java Module system.

Unfortunately, the top level directory for the `jre` or `jdk` /nix/store
entry is not a valid JAVA_HOME since it is missing that directory.

Instead it's set within `lib/openjdk`, and there is a passthru variable
set accordingly.

This fixes JRuby and follows many other derivations.
A simple search in the code-base shows that there are many other
packages that suffer this same bug.

pkgs/development/interpreters/jruby/default.nix

@@ -23,7 +23,7 @@ jruby = stdenv.mkDerivation rec {
 
      for i in $out/bin/jruby{,.bash}; do
        wrapProgram $i \
-         --set JAVA_HOME ${jre}
+         --set JAVA_HOME ${jre.home}
      done
 
      ln -s $out/bin/jruby $out/bin/ruby