fix(e2e tests): Pull branch that initially triggers gcp build for PRs (#1694)

pull branch that initially triggers gcp build for PRs

Author: hubertdeng123

cloudbuild.yaml

@@ -6,7 +6,7 @@ steps:
       - "-e"
       - "-c"
       - |
-        git clone https://github.com/getsentry/self-hosted.git
+        git clone -b $BRANCH_NAME https://github.com/getsentry/self-hosted.git
         echo 'no' > self-hosted/.reporterrors
         echo '{"version": "3.4", "networks":{"default":{"external":{"name":"cloudbuild"}}}}' > self-hosted/docker-compose.override.yml
     timeout: 60s