cachix-action-v4

• Add nixBuildArgs parameter

• Simplify how paths are pushed

  Previously we were setting up Nix build hook to make sure all paths are
  synced to cachix.

  This had a few flaws:

  • nix-daemon had to be restarted, resulting into a few seconds extra
    setup per each build
  • spurious error on macos due to nix-daemon socket not ready in time
  • significant overhead since syncing to cachix was synchronous for each
    derivation built

  Now we just look for new store paths and push those after nix-build.