cloudbuild.yaml: increase timeout to 25 minutes

Multiarch builds take considerably longer than the default (10 min)
allows.

(sync with commit e674631591)

cloudbuild.yaml

@@ -1,3 +1,4 @@
+timeout: 1500s
 steps:
   - name: gcr.io/k8s-staging-test-infra/gcb-docker-gcloud:v20211118-2f2d816b90
     entrypoint: scripts/test-infra/push-image.sh

scripts/test-infra/test-e2e.sh

@@ -19,7 +19,7 @@ i=1
 while true; do
     if make poll-images; then
         break
-    elif [ $i -ge 12 ]; then
+    elif [ $i -ge 27 ]; then
         echo "ERROR: too many tries when polling for image"
         exit 1
     fi