[bigquery] only check build image on successful build

[andrewmackett]

We were seeing unsuccessful Cloud Builds that have the images field defined were creating an error in the BigQuery notifier.

E1114 12:05:43.832522       1 notifiers.go:556] failed to run SendNotification: error parsing image manifest: error obtaining image reference: GET https://gcr.io/v2/project-id/image/manifests/tag: MANIFEST_UNKNOWN: Failed to fetch "tag"

We wouldn't expect the image to have been pushed to the repository in an unsuccessful build, so there will be no manifest for the notifier to lookup, causing the error.

This change executes the build image logic only if the build was successful.

[chitrangpatel]

/gcbrun