grant browser roles to cloud build service account

[daniel-cit]

Fixes #73

Its PR needs to wait for another PR in the foundation:

https://github.com/terraform-google-modules/terraform-example-foundation/pull/484

It can only be merged after the foundation PR is merged because the foundation app is using a foundation module

It's a good idea to open an issue first for discussion.

• [ ] Tests pass
• [ ] Appropriate changes to README are included in PR