Add basic gke cloudbuild install test

[damemi]

This adds a basic test to make sure the upstream install commands work on a GKE standard cluster. The goal is just to see that the manifests successfully apply.

[damemi]

(note for reference: I based this on the cloudbuild.yaml from https://codelabs.developers.google.com/codelabs/cloud-builder-gke-continuous-deploy in https://github.com/GoogleCloudPlatform/software-delivery-workshop/blob/main/labs/gke-progression/build/branch-cloudbuild.yaml.tmpl)

[damemi]

Update: figured out permissions/IAM issues from earlier runs. But I don't think Cloud Build has the functionality to do this the way I was thinking, specifically:

1. Step to install cert-manager prereq
2. Step to install operator
3. Cleanup step

Step 2 needs to wait for step 1 to finish (or re-try until timeout), which it doesn't look like I can do with cloud build. Then, regardless of step 1 or 2 failing, step 3 needs to always run. This also doesn't seem possible.

Probably going to have to re-work this into a basic bash script or something that we can add more logic too