workflows: add monthly flake lock update workflow

Signed-off-by: Masum Reza <masumrezarock100@gmail.com>

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

@@ -0,0 +1,21 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 1 * *' # Run monthly
+  push:
+      paths:
+        - 'flake.nix'
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: cachix/install-nix-action@v27
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@v21

.gitignore

@@ -16,6 +16,7 @@ tests/__pycache__/
 # dot-files that we don't want to ignore
 !.gitignore
 !.github/dependabot.yml
+!.github/workflows/
 !.editorconfig
 
 bcachefs-principles-of-operation.*