Fix and update Nix CI

[Zimmi48]

I just realized by analyzing a strange success in an aac-tactics overlay that Nix CI was completely broken for PRs since #107 because I stupidly forgot to update the actions/checkout@v2 step.

The second commit is a port of coq-community/coq-nix-toolbox#81. I'm taking advantage that there will need to be an update of projects relying on Nix CI to do this change in passing.

I've tested that this works on the above-mentioned aac-tactics overlay.

@palmskog I can take care of updating the 9 coq-community projects using the nix-action tag. If you know of some other projects (e.g., external to coq-community) that also use this template, please make sure they are updated as well.

[palmskog]

Thanks for catching this. I'm not aware of projects outside coq-community using the lightweight Nix CI. This is a bit of a shame, we should try to "sell" it more I think.

[palmskog]

@Zimmi48 one more project in coq-community that uses Nix CI without having the nix-action tag: https://github.com/coq-community/reglang/blob/master/.github/workflows/nix-action.yml

We probably want to check with Christian if OK to add tags like nix-action and docker-coq-action.

[Zimmi48]

@chdoc Is this OK for us to add tags to the reglang repo to denote what kind of CI is used inside and to directly push minor updates based on changes to the templates?

[chdoc]

@chdoc Is this OK for us to add tags to the reglang repo to denote what kind of CI is used inside

Assuming that by "tag" you mean what GitHub calls "topics" rather than git tags, I'm okay with this.

and to directly push minor updates based on changes to the templates?

This is fine with me too.

[Zimmi48]

Indeed, I meant topics.