Flaky `build-importer` step in Cloud Build

[cuixq]

The Go build-importer step in deployment/build-and-stage.yaml occasionally fails with the following error:

ERROR: failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: frontend grpc server closed unexpectedly

This might be a memory issue, but bumping the memory seems not fix it.

Also the Cloud Build docker image is a bit old, but not sure if we are able to upgrade it.