Add google_prod_creds to the list of auth mechanisms

This is to ensure that we don't default to using
application_default_creds when it is not set and rather
"RBE_use_google_prod_creds" variable is set. This is necessary to
rollout RBE gcert auth.

Bug: b/224446621
Change-Id: I51e3f1aefadf282240b8f861efafde6b9b61d305