nix: fix build when overriding inputs

[lucc]

I had the case in #1715 that I wanted to build alot with an update version of nixpkgs. But our code to compute the version did not evaluate for that.

[lucc]

@pazz I think that you did not use nix so this might be difficult for you to test/review.

But I can not approve my own PR, so what do you think is the best course of action here?

[pazz]

let's wait for a copilot review, then amend/ignore it, and you can merge?

[Copilot]

Pull request overview

This PR updates the Nix flake to make version computation resilient when the flake is built with overridden inputs (e.g., different nixpkgs), and refreshes the CI Nix installer action.

Changes:

• Compute a version once at the flake output level with a fallback when shortRev/dirtyShortRev are unavailable.
• Refactor the Nix packaging to build a Python package (buildPythonPackage) and then produce an application via toPythonApplication, plus add a theme-test package.
• Bump cachix/install-nix-action used in CI.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
flake.nix	Adjusts version derivation logic and refactors Python/Nix packaging; adds a theme-test derivation.
.github/workflows/check.yml	Updates the GitHub Action used to install Nix during CI jobs.

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[lucc]

@pazz yes I can merge in theory but only if the PR has one approving review (not from me if I am the author). That is the branch protection rule for master. And that was the reason for calling out to you here in the first place. I think I need the approval from you (as we are the only people with commit rights, no?)

[pazz]

here we go :) thanks