ci: check flake in build job

.github/workflows/build.yml

@@ -54,6 +54,8 @@ jobs:
       with:
         name: ${{ matrix.cachixName }}
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - name: Check nix
+      run: nix flake check --show-trace
     - name: Build nix packages
       run: nix develop .#ci -c nix-build-uncached ci.nix -A cacheOutputs
     - name: Trigger NUR update