impersonate service account and type jwt does not seem to work well together

I'm trying something similar to:
```
oauth2l fetch --refresh --impersonate-service-account='xxxyyy@gke-accounts.iam.gserviceaccount.com' --type jwt --audience="https://google.com"
```
And I see the following error:

```
google: could not parse JSON key: google: read JWT from JSON credentials: 'type' field is "authorized_user" (expected "service_account")
```

It seems like no impersonated credentials are used in the JWT code path.

What I would like is the impersonated identity token (Patching JWTTokenSource with https://pkg.go.dev/google.golang.org/api/impersonate#IDTokenSource seems to give me what I'm looking for)