Although when I changed that I got another error:

polling operation status: googleapi: Error 404: Requested entity was not found.

But the Pulumi resource was created (and couldn't be updated):

~   ├─ google-native:cloudbuild/v1:Build  prod-auth-server-image  updated (0.01s)     1 warning
 ~   └─ google-native:cloudbuild/v1:Build  prod-bastion-image      updated (0.03s)     1 warning


Diagnostics:
  google-native:cloudbuild/v1:Build (prod-bastion-image):
    warning: update is currently undefined for resource: "google-native:cloudbuild/v1:Build". v1/projects/production/locations/us-west2/builds/c9bce8dd-5065-403b-8904-511dfc9e75ad will not be updated

  google-native:cloudbuild/v1:Build (prod-auth-server-image):
    warning: update is currently undefined for resource: "google-native:cloudbuild/v1:Build". v1/projects/production/locations/us-west2/builds/a89d2819-2ace-4008-8efc-72db2186a7c4 will not be updated

Overall, the correct change is hard to find because there's a lot of generated code checked into the source repository (!!!)

Oh, polling fails because it's also using the base64 encoded UUID string:

polling operation status (URL=<https://cloudbuild.googleapis.com/v1/operations/build/production/N2QwYTU2ZjItYzlhOC00MGRiLTgzYzQtZDJjZWE2YzE4N2Yz>): googleapi: Error 404: Requested entity was not found.

There seems to be no equivalent functionality in GCP Classic.

Let me know if I can help resolve this by putting you in touch with GCP.