fix(symbolicator): check source credentials and not the credential token (#92024)

This PR fixes a bug where it would always report that no token can be
obtained.
The reason is that no refresh is being performed on the
`source_credentials`, so it does not have a `token` property yet and the
check will always return `False`.

Author: Litarnus

src/sentry/lang/native/sources.py

@@ -616,7 +616,7 @@ def get_gcp_token(client_email):
     # Fetch the regular credentials for GCP
     source_credentials, _ = google.auth.default()
 
-    if source_credentials.token is None:
+    if source_credentials is None:
         return None
 
     # Impersonate the service account to give the token for symbolicator a proper scope