clojure: move exec.jar to libexec

[bennyandresen]

Motivation for this change

this change fixes #103972

I've also tested the current 20.09 version of clj-new and it fails for a different reason, so this commit should only be an improvement.

Whoever merges this, could you please also backport it to 20.09?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

/marvin opt-in
/status needs_reviewer

cc:
@will08rien

[marvin-mk2]

Hi! I'm an experimental bot. My goal is to guide this PR through its stages, hopefully ending with a merge. You can read up on the usage here.

[bennyandresen]

closing in favor of #104190