ci: allow both builder cache flavors to warm

Author: daviszhen

.github/workflows/image-build.yaml

@@ -224,8 +224,9 @@ jobs:
     continue-on-error: true
     environment: ci
     runs-on: ubuntu-22.04
-    # Each warm flavor is roughly a full-tree compile.
-    timeout-minutes: 120
+    # The coverage and plain Go flavors are each full-tree compiles; leave
+    # enough room for both nightly warms plus the UT cache seed steps.
+    timeout-minutes: 180
     steps:
       - name: Checkout
         uses: actions/checkout@v6