Add flake check workflow

.gitea/workflows/push.yaml

@@ -0,0 +1,22 @@
+name: Push workflow
+
+on: push
+
+jobs:
+  flake-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v12
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+          extra_conf: |
+            experimental-features = nix-command flakes
+
+      - name: Flake check
+        run: nix flake check