Nix CI reverse dependency compatibility testing

[4ever2]

This PR attempts to fix the current Nix CI and sets up reverse dependency compatibility testing for all the libraries in nixpkgs that depend on MetaCoq.

Currently, Nix CI is only configured for the 8.19 branch.

[4ever2]

The Nix CI works again now. The CI is quite slow because caching is disabled on pull requests that modify the workflow files, caching should work when merged assuming that the CACHIX_AUTH_TOKEN secret still exists and is valid.

I will set up CI for the 8.20 and main branches in separate PRs later if this is something that we want.

[4ever2]

@yforster does this look good to you?
I have not yet added CertiCoq as a reverse dependency in CI since CertiCoq doesn't use Nix. I will look into adding it in a later PR.

[yforster]

Looking at the actions, I see that both the ElmExtraction and RustExtraction targets run for 33m. Does that mean they literally rebuild MetaCoq, or do they just wait for the MetaCoq job to finish? From the log it seems like they are rebuilding, meaning we build MetaCoq 3 times or so per CI run now?

[4ever2]

Yes, they are rebuilding MetaCoq since caching hasn't been enabled yet. It should work once merged assuming that the CACHIX_AUTH_TOKEN is still valid.
The reason why it doesn't work currently is because caching is disabled for security reason on pull requests that modify CI.