Make sure `auth-integration-test` instructions are up to date

We have instructions to run the integration tests in a test project that is not `rust-auth-testing`:

https://github.com/googleapis/google-cloud-rust/tree/main/src/auth/integration-tests#in-your-own-test-project

They are stale. We need to update them. (Or we could remove them, but that seems unnecessarily sad. I would not approve such a PR).