Allow import-from-derivation so Haskell projects can build with this action

[ParetoOptimalDev]

I think that two things have to happen:

• allow putting allow-import-derivation as a config option
• allow specifying number of builders to 1 for IFD here

[w4]

I've been looking into this myself, turns out you can already set IFD as a config option:

jobs:
  build:
    uses: nixbuild/nixbuild-action/.github/workflows/ci-workflow.yml@v17
    secrets:
      nixbuild_token: ${{ secrets.NIXBUILD_TOKEN }}
    with:
      nix_conf: |
        allow-import-from-derivation = true

[w4]

IFD madness continues after the above when trying to run aarch64-linux builds from GH runners because of the call to nix flake show. It's honestly not worth bothering. I've just ended up using pkgs.haskellPackages.mkDerivation directly. Works perfectly when you're not in the haskell/flake hellhole.

Thanks for the work on this action & nixbuild as a whole @rickynils