Flake in `auth` integration tests

First time I see this:

```
failures:
 
---- driver::run_id_token_adc_impersonated stdout ----
 
thread 'driver::run_id_token_adc_impersonated' (7342) panicked at tests/integration-auth/src/lib.rs:601:10:
failed to get id token: CredentialsError { is_transient: true, message: Some("failed to fetch token, body=<{\n \"error\": {\n \"code\": 500,\n \"message\": \"Unable to extract the resource from the request.\",\n \"status\": \"INTERNAL\"\n }\n}\n>"), source: Some(reqwest::Error { kind: Status(500, None), url: "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/impersonation-target@rust-auth-testing.iam.gserviceaccount.com:generateIdToken" }) }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
```
https://pantheon.corp.google.com/cloud-build/builds;region=us-central1/09072aa8-ba52-4d5b-9522-92c92cc0f047?project=rust-auth-testing&e=SqlStudioMysqlLaunch::SqlStudioMysqlEnabled&mods=allow_workbench_image_override

It may be a legitimate transient error that needs some kind of retry loop.