coqPackages.coq-elpi: enable override of ocamlPackages.elpi

[proux01]

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[proux01]

@vbgl in CI like coq-elpi or hierarchy-builder, we often need to override the version of ocamlPackages.elpi, which we can currently only do for coqPackages.coq-elpi master, this hack enables it

CI "green":

• here
• on Test enable override of ocamlPackages.elpi coq-community/coq-nix-toolbox#311 checking that it doesn't break anythin
• on [do not merge] [CI] Test nix elpi math-comp/hierarchy-builder#501 checking that it indeed works

[proux01]

Thanks!