This adds a nix CI job to build the flake in debug mode for
aarch64-darwin and x86-linux. For now this job will only run when the
`run-nix` label is added to a PR.

The CI job doesn't push to cachix for now, so every build is a clean
build.

I also added a condition to the garbage collection step so it only runs
when the nix store is >50GB.

Release Notes:

- N/A