Only bake warpbuild snapshot from stable

There's a recent breaking change that deprecated dashboard-based snapshot enablement, and is breaking our CI:
https://www.warpbuild.com/docs/ci/changelog/2026-june#june-8-2026

> Legacy dashboard-based snapshot enablement has been removed. Runners previously configured with snapshots enabled from the dashboard will no longer use snapshots. To continue using snapshot runners, migrate to the label-based configuration by adding snapshot.enabled=true or snapshot.key=<alias> to your runs-on labels. See the [Snapshot Runners docs](https://www.warpbuild.com/docs/ci/features/snapshot-runners) for migration details.


  


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

.github/workflows/warpbuild-ubuntu-latest-snapshot.yml

@@ -5,8 +5,11 @@ on:
   schedule:
     # Every week (Sunday at 00:00 UTC)
     - cron: "0 0 * * 0"
-  pull_request:
-    branches: [stable, unstable]
+  push:
+    # The warpbuild snapshot is global across all branches, so only `stable`
+    # bakes it. This avoids non-stable branches (and Mergify merge-queue PRs)
+    # clobbering the shared snapshot and racking up duplicate bakes.
+    branches: [stable]
     paths:
       - '.github/workflows/warpbuild-ubuntu-latest-snapshot.yml'
 
@@ -16,7 +19,7 @@ concurrency:
 
 jobs:
   bake:
-    runs-on: warp-ubuntu-latest-x64-8x
+    runs-on: warp-ubuntu-latest-x64-8x;snapshot.enabled=true
     steps:
       - name: Install system deps
         run: |