[GCP] ADC authentication not providing project ID in integration test

[seankimkdy]

Due to golang/oauth2#241, we are unable to retrieve the project through oauth2's FindDefaultCredentials when using application default credentials (ADC) as we do for GCP integration tests. This is why we need to set the environment variable PARAGLIDER_GCP_PROJECT. Once the oauth2 issue mentioned above is resolved, the integration test setup should be changed to automatically find the project id.

[divega]

Triage: this is blocked on the issue linked.