Set up CI to automatically update Nixpkgs (#12)

Yet untested; let's see it work or fail.

.github/workflows/update.yml

@@ -0,0 +1,26 @@
+name: Update Nixpkgs pin
+on:
+  workflow_dispatch: # Allows triggering manually
+  schedule:
+    - cron: '47 14 * * *' # runs every day at 14:47 UTC (chosen somewhat randomly)
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v26
+
+      - name: update
+        run: |
+          nix-build -A pkgs.npins
+          result/bin/npins update nixpkgs
+
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@v6
+        with:
+          commit-message: "Automated Nixpkgs update"
+          branch: auto-nixpkgs-update
+          title: "Automated Nixpkgs update"
+          body: "Update the pinned Nixpkgs automatically"

default.nix

@@ -39,6 +39,9 @@ build // {
 
   inherit build;
 
+  # Used by CI and good for debugging
+  inherit pkgs;
+
   shell = pkgs.mkShell {
     env.NIX_CHECK_BY_NAME_EXPR_PATH = toString runtimeExprPath;
     env.NIX_PATH = "test-nixpkgs=${toString testNixpkgsPath}:test-nixpkgs/lib=${toString nixpkgsLibPath}";