ci: Add nix Flake check to flake.yml

The nix flake check Command Tests rather the nix package builds in all supported systems and checks formatting.

Signed-off-by: naŭ glenda <plan9git@proton.me>

.github/workflows/flake.yml

@@ -15,3 +15,5 @@ jobs:
         uses: DeterminateSystems/nix-installer-action@v4
       - name: Build default package
         run: nix build
+      - name: Nix Flake Check
+        run: nix flake check --all-systems