buildPython*: simplify check-related attribute inheritance

[ShamrockLee]

This PR simplifies the inheritance of check-related attributes. It essentially pass these arguments directly whenever attrs.doCheck is true or unspecified.

This PR is implemented with a local helper function getExistingAttrs, which is like getAttrs but ignores non-existing attributes. This helps reduce boilerplate codes, and subsequent PRs will also utilize such helper function.

This can be seen as the non-controvertial, backward-compatible part of #376060.

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.

---

Add a 👍 reaction to pull requests you find important.

[wolfgangwalther]

Just read through it, didn't build, but LGTM.

[ShamrockLee]

@ofborg build python3Packages.pytestCheckHook.tests

[ShamrockLee]

OfBorg's Linux machines build python3Packages.pytestCheckHook.tests successfully, while x86_64-darwin times out after 3600 seconds.

I'll merge this PR.