openjdk8: add `setJavaClassPath`-hook to jdk as well

This hook got removed from JDK[1], however without this hook,
the classpath in a Java-build isn't created anymore which caused
several[2][3] broken packages.

[1] https://github.com/NixOS/nixpkgs/pull/65247/files#r324459267
[2] https://hydra.nixos.org/build/100896633
[3] https://hydra.nixos.org/build/100895668

ZHF #68361