ci: nix-flake update workflow

Introduces a GitHub Actions workflow
to update the flake.lock file for nixpkgs.
This allows us to detect breaking changes in dependencies,
preventing peerswap from failing to build.

Author: YusukeShimizu

.github/workflows/nix-flake.yml

@@ -0,0 +1,26 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 * * *' # runs daily at 00:00
+  push:
+    branches:
+      - master
+  pull_request:
+    branches: [master]
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: "Update flake.lock" # Title of PR to be created
+          pr-labels: |                  # Labels to be set on the PR
+            dependencies
+            automated