Guard publishing to CIPD around a check if it tag already exists.

I tested the `api.cipd.search()` call on LED.

Bug: https://github.com/flutter/flutter/issues/55082
Change-Id: I534d1129f33006dfc42a07dd2ac5992c41df54bb
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/4345
Reviewed-by: Tong Wu <wutong@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Christopher Fujino <fujino@google.com>