cargoCheckHook: proof of concept of fix for the test-threads issue

[lucasew]

Some tests are failing because an extra arg is being passed to set the amount of threads for cargo test.

This is a proof of concept with the assumption that the arguments passed somehow are stepping on toes of
other argument parsers. Assuming that this amount of threads can also be passed via RUST_TEST_THREADS,
we could achieve the same behaviour and avoid causing this problem.

• cargoCheckHook: pass test-threads using an environment variable
• treewide: reenable tests of packages affected with the test-threads issue

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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[lucasew]

I plan to first test with the affected packages, and if I get success with them I rebase this PR to staging because of the amount of rebuilds. That way I do not have to run reviewd in rounds because of the amount of packages.

[lucasew]

The PoC works, but there are failing tests not related to the issue being fixed in this PR.