ci: add DeterminateSystems/magic-nix-cache-action

[thiagokokada]

Description

There is zero reason why the current CI for home-manager is that slow. We can cache the CI results and basically get a very fast CI (e.g.: only run tests for the changed code) as long as we store the /nix/store somewhere and restore before running the tests.

This PR adds DeterminateSystems/magic-nix-cache-action to do so.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

[rycee]

We actually used to have caching but I removed it at some point (#5093) because it was actually slowing things down.

Maybe it's worth trying to re-enable it. But I suspect it still is not a big win.

Would be great to make the tests have a more reasonable runtime so well worth investigating more deeply. E.g., how to reduce the number of evaluations of Nixpkgs.

[thiagokokada]

Maybe it's worth trying to re-enable it. But I suspect it still is not a big win.

I amended the commit to re-trigger the tests, let's see. I imagine that this new run should be significantly faster than the previous one (that took ~43 min), but if there is no change it wouldn't be worth it.

[thiagokokada]

Took basically the same time, yes, it seems it is not worth it at all.