The startup script used by jconsole (/bin/profile.ijs) doesn't try very hard to understand the file hierarchy it lives in, and just did not work with NixOS. For reasons unknown, it Just Worked for some time, but finally broke with an actual error message every time you launch jconsole in recent versions of nixpkgs.
This commit just overwrites all the heuristics J uses to find out where its installation files are, to point where they actually are put by the installer.
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