Support environment variables in gcp_credentials: workload_identity_provider: etc

[anarazel]

Hi,

right now it is not supported to configure gcp_credentials etc via environment variables. It'd be useful to support doing so, to allow different forks of a repository to use different workload identity providers.

I searched through the cirrus_cli code, without finding relevant references to gcp_credentials/workload_identity_provider/.. - presumably that is the problem?

Regards,

Andres