[fuchsia-ctl] Fix checkout path

The checkout dir was broken due to the way that api.git calculates it. See https://ci.chromium.org/p/flutter/builders/try/fuchsia_ctl/3.

This change fixes the issue by specifying the checkout path explicitly. See https://chromium-swarm.appspot.com/task?id=4c514fb5cd68b110

Bug: https://github.com/flutter/flutter/issues/56138

Change-Id: Ia26b6ec9fcae655b169016a14927257fc95be77a
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/3045
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Tong Wu <wutong@google.com>