Right now, the only environment variable available is the same
environment variable that will be picked up by the GCP provider. Users
would like to be able to store state in separate projects or accounts or
otherwise authenticate to the provider with a service account that
doesn't have access to the state. This seems like a reasonable enough
practice to me, and the solution seems straightforward--offer an
environment variable that doesn't mean anything to the provider to
configure the backend credentials. I've added GOOGLE_BACKEND_CREDENTIALS
to manage just the backend credentials, and documented it appropriately.
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