Document GCP auth with environment variables

[VenelinMartinov]

What happened?

https://www.pulumi.com/registry/packages/gcp/installation-configuration/ has nothing on using GOOGLE_OAUTH_ACCESS_TOKEN for authentication. This seems to have caused some confusion on how OIDC can be configured: #1693

We support authenticating via the GOOGLE_OAUTH_ACCESS_TOKEN env var, so we should have it documented.

Example

the following ESC config should work - this avoids saving the token in the state

values:
  gcp:
    login:
      fn::open::gcp-login:
        project: -
        oidc:
          workloadPoolId:-
          providerId: -
          serviceAccount: -
  environmentVariables:
    GOOGLE_PROJECT: PROJECT
    GOOGLE_OAUTH_ACCESS_TOKEN: TOKEN

Output of pulumi about

.

Additional context

No response

Contributing

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[iwahbe]

This feels like an instance of pulumi/pulumi-terraform-bridge#1585.

[VenelinMartinov]

I believe https://www.pulumi.com/registry/packages/gcp/installation-configuration/ is hand-authored, so it'd not be fixed by the bridge issue.