Fix presubmit docs.

The failure reason is we were trying to upload docs for try jobs.

Change-Id: I68ea7676e55d8a62a7b40b0b43d5244e408a4819
Bug: https://github.com/flutter/flutter/issues/68111
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/7520
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Dan Field <dnfield@google.com>
Reviewed-by: Dan Field <dnfield@google.com>