improve cloud build failure monitoring & alerting rule

[hyangah]

Since cloud build does not export success/failure metrics (https://issuetracker.google.com/issues/248609607)
the current rule uses the cloud build log and creates an incident when it sees "ERROR" level log message.
Unfortunately, that doesn't distinguish cases like

• The internal CB trigger misbehaves and creates duplicate builds on the same commit simultaneously. In that case, only one of them will succeed thank to our own deployment locking mechanism. But the failed builds will fire alerts. (e.g. build id = b3d258c6).

• When build is cancelled in favor of the following build request (e.g. cls submitted back-to-back??), the cancelled build produces "ERROR" level log and triggers alerts. (e.g. build id = d451fa13).