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.

.github/workflows/nix-flake.yml

@@ -0,0 +1,25 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 * * *' # runs daily at 00:00
+  push:
+    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"
+          base: "master"
+          pr-labels: |
+            dependencies
+            automated

flake.nix

@@ -1,11 +1,6 @@
 {
   inputs = {
-    # Pinning to revision 4f0dadbf38ee4cf4cc38cbc232b7708fddf965bc 
-    # - cln v24.11
-    # - lnd v0.18.3-beta
-    # - bitcoin v28.0
-    # - elements v23.2.4
-    nixpkgs.url = "github:NixOS/nixpkgs/4f0dadbf38ee4cf4cc38cbc232b7708fddf965bc";
+    nixpkgs.url = "github:NixOS/nixpkgs";
     flake-utils.url = "github:numtide/flake-utils";
     # blockstream-electrs: init at 0.4.1 #299761
     # https://github.com/NixOS/nixpkgs/pull/299761/commits/680d27ad847801af781e0a99e4b87ed73965c69a