CI builds for PRs

[cleeyv]

The ngi0 hydra is configured to build all the packages in ngipkgs: https://hydra.ngi0.nixos.org/jobset/ngipkgs/main#tabs-jobs however, this can only be run for specific branches, not for new PRs, hence the need for a parallel CI solution.

CI has been implemented for PRs using github actions: 4dfa911
Currently it is running nix flake check, which confirms that everything evaluates, and runs all tests, but provides no way to check that all packages build correctly. This is a feature that has been requested more than once over the past few years:
NixOS/nix#3811
NixOS/nix#7165
The first request noted that it would be "very useful in the context of a monorepo". In the comments of the second request, there are some workarounds mentioned, such as adding all packages as "checks" so that they will be built by nix flake check, or using symlinkJoin to create a single derivation containing all packages which can then be set to the default package for the flake and more easily built in CI: https://www.tweag.io/blog/2022-09-22-rust-nix/

[cleeyv]

I tried the symlinkJoin method first but couldn't get it to work. Then I tried adding it to the checks, and once I figured out how to do it, it was very simple: 99ba38f
I got inspiration from this implementation of a package build check, which seemed to be also having problems with unwanted darwin builds: ngi-nix/pgp-milter@cae9fd3

The result when I pushed the new commit was a CI run that was 8 minutes long (compared to the previous 3 minutes without builds), with logs that included builds of the packages! https://github.com/ngi-nix/ngipkgs/actions/workflows/ci.yaml

The remaining step is testing to confirm that this works as intended with a PR.

[mightyiam]

Brilliant.

[cleeyv]

Confirmed that it is working as intended on PRs: #20