feat: enable automerge for flake.lock update PRs

Automatically enable rebase automerge on PRs created by the
update-flake-lock workflow using GH_TOKEN_FOR_UPDATES secret.

Author: typedrat

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

@@ -6,13 +6,19 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/determinate-nix-action@v3
-      - name: Update flake.lock
+      - id: update
+        name: Update flake.lock
         uses: DeterminateSystems/update-flake-lock@main
         with:
           pr-labels: |-
             dependencies
             automated
           pr-title: 'chore: update flake.lock'
+      - env:
+          GH_TOKEN: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
+        if: steps.update.outputs.pull-request-number != ''
+        name: Enable automerge
+        run: gh pr merge --auto --rebase ${{ steps.update.outputs.pull-request-number }}
 name: Update flake.lock
 "on":
   schedule:

dev/flake-module.nix

@@ -166,13 +166,22 @@
                   uses = "DeterminateSystems/determinate-nix-action@v3";
                 }
                 {
+                  id = "update";
                   name = "Update flake.lock";
                   uses = "DeterminateSystems/update-flake-lock@main";
                   with_ = {
                     pr-title = "chore: update flake.lock";
                     pr-labels = "dependencies\nautomated";
                   };
                 }
+                {
+                  name = "Enable automerge";
+                  if_ = "steps.update.outputs.pull-request-number != ''";
+                  run = "gh pr merge --auto --rebase \${{ steps.update.outputs.pull-request-number }}";
+                  env = {
+                    GH_TOKEN = "\${{ secrets.GH_TOKEN_FOR_UPDATES }}";
+                  };
+                }
               ];
             };
           };