ci: move GitHub-hosted jobs to createos runners

Replaces `runs-on: ubuntu-latest` with the `createos` label so these jobs run
on the CreateOS ephemeral microVM runners (createos-sandbox-ghar) instead of
billable GitHub-hosted Linux runners.

Author: pratikbin

.github/workflows/ci.yaml

@@ -14,7 +14,7 @@ jobs:
   image-build-push:
     permissions:
       contents: read
-    runs-on: ubuntu-latest
+    runs-on: createos
     steps:
       - name: checkout repository
         uses: actions/checkout@v6