cleanup: use secrets manager instead of encrypted secrets for GH tokens

Currently the GH token used for the CI steps in CloudBuild is encrypted and is kept in the cloudbuild yaml files
However, there is a suggestion to move this out of here and use CloudBuild-SecretsManager integration to store and access the token from there
This was tried in PR #203 . However, it didn't work
The reason for it not working seems to be that CloudBuild doesn't have the right permissions to access the secrets
Regardless of what level of permission is given this seems to be not working
Thus, an internal bug b/267166557 was openned against the documentation to provide more clarity as to how to get this working.

GoogleCloudPlatform / point-of-sale