.github/workflows: separate building hosts and packages

It is a better idea to make them separate at this point since the
packages need to be immediately sent to the binary cache.

.github/workflows/build.yml

@@ -1,7 +1,7 @@
-name: "Build packages"
+name: "Building derivations"
 on: [pull_request, push]
 jobs:
-  build:
+  build-custom-packages:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
@@ -16,5 +16,14 @@ jobs:
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
       - name: Build custom packages
         run: nix shell nixpkgs#nix-build-uncached --command nix-build-uncached ./pkgs
+
+  build-hosts:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v16
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
       - name: Build NixOS configurations
-        run: nix build .#nixosConfigurations.ni.config.system.build.toplevel
+        run: |
+          nix build .#nixosConfigurations.ni.config.system.build.toplevel