CI: add reusable actions

.github/workflows/nix-update-wlroots.yml

@@ -0,0 +1,34 @@
+name: Nix
+
+on:
+  workflow_call:
+    secrets:
+      PAT:
+        required: true
+
+jobs:
+  update:
+    name: wlroots
+    runs-on: ubuntu-latest
+    steps:
+    - name: Clone repository
+      uses: actions/checkout@v3
+      with:
+        token: ${{ secrets.PAT }}
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v22
+      with:
+        install_url: https://nixos.org/nix/install
+        extra_nix_config: |
+          auto-optimise-store = true
+          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+          experimental-features = nix-command flakes
+
+    - name: Update lockfile
+      run: nix/update-wlroots.sh
+
+    - name: Commit
+      uses: stefanzweifel/git-auto-commit-action@v4
+      with:
+        commit_message: "[gha] Nix: update wlroots"