Run flake check before build

Author: ranfdev

.github/workflows/build-flatpak.yml

@@ -15,5 +15,5 @@ jobs:
       with:
         name: ranfdev-geopard
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-    - run: nix build
-    - run: nix flake check
+    - run: nix flake check
+    - run: nix build