feat: Add nix-flake-check

.forgejo/workflows/nix-flake-check.yaml

@@ -0,0 +1,23 @@
+name: Flake Check
+
+on:
+  workflow_call:
+
+jobs:
+  check:
+    runs-on: ubuntu-latest-amd64
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install Nix
+      uses: https://github.com/DeterminateSystems/nix-installer-action@v4
+      with:
+        init: none
+        planner: linux
+        github-token: null
+    - name: Setup Attic
+      uses: https://github.com/ryanccn/attic-action@v0
+      with:
+        endpoint: ${{ secrets.ATTIC_ENDPOINT }}
+        cache: ${{ secrets.ATTIC_CACHE }}
+        token: ${{ secrets.ATTIC_TOKEN }}
+    - run: nix flake check --keep-going