enable nix flake check in GitHub Action

Author: peterbecich

.github/workflows/nix-flake.yml

@@ -29,4 +29,7 @@ jobs:
         name: hackage-server
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
     - run: nix build
-    # - run: nix flake check
+
+    # allow error until Flake check is improved
+    - continue-on-error: true
+      run: nix flake check