Re-add `nix flake check` CI job

[mitchmindtree]

A reminder to re-add the following job once this upstream issue is resolved:

  nix-flake-check:
    needs: cancel-previous-runs
    runs-on: ubuntu-latest
    steps:
      - uses: actions/[email protected]
      - uses: cachix/install-nix-action@v16
        with:
          nix_path: nixpkgs=channel:nixos-unstable
      - uses: cachix/cachix-action@v10
        with:
          name: mitchmindtree-fuellabs
          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
      - run: nix flake check --no-update-lock-file

Currently, we still check builds for each package, but the nix flake check is useful for quickly checking the validity of the flake itself for all targets.