ci: add flake check

.github/workflows/build.yml

@@ -1,6 +1,16 @@
 name: "Building derivations"
 on: [pull_request, push]
 jobs:
+  check-outputs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v16
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - name: Check flake outputs
+        run: NIXPKGS_ALLOW_INSECURE=1 nix --impure flake check
+
   build-custom-packages:
     runs-on: ubuntu-latest
     steps: