Move Build GhosttyKit to Depot to avoid self-hosted concurrency cancellations (#713)

build-ghosttykit shared the self-hosted-build concurrency group with
CI tests, causing one to get cancelled when both trigger on the same
push. Most runs are no-ops (xcframework already exists), so Depot is
a good fit. Eliminates the red X on pushes to main.

.github/workflows/build-ghosttykit.yml

@@ -8,12 +8,9 @@ on:
 
 jobs:
   build-ghosttykit:
-    # Never run self-hosted jobs for fork pull requests.
+    # Never run Depot jobs for fork pull requests (avoid billing on external PRs).
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
-    runs-on: self-hosted
-    concurrency:
-      group: self-hosted-build
-      cancel-in-progress: false
+    runs-on: depot-macos-latest
     steps:
       - name: Checkout
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4