actions: add flake checks

.gitea/workflows/flake.yaml

@@ -0,0 +1,30 @@
+name: flake
+
+on:
+  push:
+    branches:
+      - '**'
+    tags-ignore:
+      - '**'
+
+jobs:
+  flake:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b # v4.1.4
+      - name: Prepare for Nix installation
+        run: |
+          apt-get update
+          apt-get install -y sudo
+      - uses: cachix/install-nix-action@8887e596b4ee1134dae06b98d573bd674693f47c # v26
+      - name: lint
+        run: |
+          nix fmt
+          git diff --exit-code
+      - name: flake check
+        run: nix flake check --all-systems
+        timeout-minutes: 10
+      - name: nix logs
+        if: failure()
+        run: nix log .
+