Drop the use of credentials files

[puerco]

In the latests gcp sdk functions, the WithCredentialsFile option is deprecated.

The derprectation is now causing errors on our lint jobs, I've silenced them for now, but the real fix is to drop the use of credential files entirely:

• Now that the promoter is supposed to get a revamp, let's plan for a stable identity in the jobs so that we can properly impersonate the signer service account from the promoter workload identity.

• We also need to revamp the e2e tests to drop the use of the CIP_E2E_KEY_FILE credential file.

I think those are the places where json creds are used. If there are more, feel free to add them to this issue.