.github/workflows: add automation for updating dotfiles input

.github/workflows/automate-dotfiles-input-update.yml

@@ -0,0 +1,30 @@
+name: "Automated flake input update for my dotfiles"
+on:
+  schedule:
+    # Every day.
+    - cron: '30 0 * * *'
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v16
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - name: Update flake inputs
+        run: |
+          nix shell nixpkgs#git
+
+          # This is set by the GitHub Actions bot just to make identifying which commits are automated.
+          git config user.name 'github-actions[bot]'
+          git config user.email 'github-actions[bot]@users.noreply.github.com'
+
+          nix flake lock --update-input 'dotfiles' --commit-lock-file --commit-lockfile-summary "flake.lock: update dotfiles"
+      - name: Build NixOS configurations that uses the dotfiles
+        run: nix build .#nixosConfigurations.ni.config.system.build.toplevel
+      - name: Push updates to remote
+        run: |
+          nix flake lock --commit-lock-file
+          git remote remove origin
+          git remote add origin https://${{ secrets.GITHUB_TOKEN }}@github.com/foo-dogsquared/nixos-config.git
+          git push --set-upstream origin master --tags