fix: Correctly set the credentials token source and the WithIAMAuthN opt (#2544)

This fixes a bug in how the auth proxy handled the configuration when the `--impersonate-service-account` flag and the `auto-iam-auth` instance configuration parameter are set at the same time. This is especially important to make the Cloud SQL Proxy Operator work correctly, see GoogleCloudPlatform/cloud-sql-proxy-operator#719

Fixes #2542