Automatic merge from submit-queue

Set GOOGLE_APPLICATION_CREDENTIALS after activating

https://developers.google.com/identity/protocols/application-default-credentials

We should set this explicitly as https://golang.org/pkg/os/user/#User (which the oauth libraries use: [sdk.go] - [default.go] via [CreateGCECloud]) will ignore any `$HOME` overrides.

[CreateGCECloud]: https://github.com/kubernetes/kubernetes/blob/fa95788e560de7575c851d87d76c664b3753a7a6/test/e2e/e2e.go#L75
[sdk.go]: https://github.com/kubernetes/kubernetes/blob/fa95788e560de7575c851d87d76c664b3753a7a6/vendor/golang.org/x/oauth2/google/sdk.go#L165
[default.go]: https://github.com/kubernetes/kubernetes/blob/fa95788e560de7575c851d87d76c664b3753a7a6/vendor/golang.org/x/oauth2/google/default.go#L101

Mitigates https://github.com/kubernetes/kubernetes/issues/28514 and related to https://github.com/kubernetes/test-infra/issues/266