Ensure tarball always builds when PR is green

I just had a case where I blocked the channel build accidently even though the PR was green: https://github.com/NixOS/nixpkgs/pull/52454

Would it be feasible to run some more checks, to make sure that the tarball always builds?
Building release.nix/tarball might be too much (requires lots of RAM for full jobset evaluation), but perhaps the following would work as check? (taken from pkgs/top-level/make-tarball.nix)

    # Check that all-packages.nix evaluates on a number of platforms without any warnings.
    for platform in i686-linux x86_64-linux x86_64-darwin; do
        header "checking Nixpkgs on $platform"

        nix-env -f . \
            --show-trace --argstr system "$platform" \
            -qa --drv-path --system-filter \* --system \
            "''${opts[@]}" 2>&1 >/dev/null | tee eval-warnings.log

        if [ -s eval-warnings.log ]; then
            echo "Nixpkgs on $platform evaluated with warnings, aborting"
            exit 1
        fi
        rm eval-warnings.log

        nix-env -f . \
            --show-trace --argstr system "$platform" \
            -qa --drv-path --system-filter \* --system --meta --xml \
            "''${opts[@]}" > /dev/null
        stopNest
    done

I can also implement this, if I can get a pointer where (which file) to start implementing this.

Start by tracing the PR-green and channel-build workflow, then compare it with release.nix/tarball and the checks shown from pkgs/top-level/make-tarball.nix. Determine where tarball evaluation can run without full jobset evaluation; done means a green PR cannot block the channel build because the tarball check was omitted, and failures are reported.