ci: run flake check before building stuff

.github/workflows/cachix.yaml

@@ -9,9 +9,27 @@ on:
       - 'flake.lock'
       - '.github/workflows/*'
 jobs:
+  flake-check:
+    name: Flake check
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - uses: cachix/install-nix-action@v16
+
+    - uses: cachix/cachix-action@v10
+      with:
+        name: alarsyo
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+        extraPullNames: "nix-community"
+
+    - name: Build package
+      run: nix flake check
+
   build-pkgs:
     name: Nix packages
     runs-on: ubuntu-latest
+    needs: [ flake-check ]
 
     strategy:
       fail-fast: false