Add full flake check in CI and monthly flake.lock maintence job

Add flake check in CI workflow and weekly flake.lock update and check after lock updates

Author: Mshehu5

.github/workflows/flake-check.yml

@@ -0,0 +1,24 @@
+name: Continuous integration
+
+on:
+  push:
+  pull_request:
+    types: [opened, synchronize, reopened]
+    paths-ignore:
+      - "flake.lock"
+
+jobs:
+  flake-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+
+      - name: Enable magic Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@main
+
+      - name: Run full flake check
+        run: nix flake check -L

.github/workflows/flake-maintenance.yml

@@ -0,0 +1,33 @@
+name: Flake maintenance
+
+on:
+  schedule:
+    - cron: "0 14 * * 1" # weekly at 14:00 UTC
+  workflow_dispatch:
+
+jobs:
+  update-flake-lock:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+
+      - name: Enable magic Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@main
+
+      - name: Validate recreated lockfile
+        run: nix flake check -L --recreate-lock-file --no-write-lock-file
+
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: "chore: update flake.lock"
+          commit-msg: "chore: update flake.lock"
+          token: ${{ secrets.GITHUB_TOKEN }}
+          nix-options: --offline