Support environment variables in gcp_credentials: workload_identity_provider: etc

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

Search the cirrus_cli code for gcp_credentials, workload_identity_provider, and related configuration handling to locate the relevant entry point. Determine which credential settings should accept environment variables, then verify that different repository forks can select different workload identity providers through those variables.

Written by the indexing model from the issue text.