GitLab

Clear it before any tests begin.
Clear it again after TestIsSecureGitAllowProtocol sets it.

Fixes #17700.

Change-Id: I6ea50d37f8222d8c7c9fee0b1eac3bbdfb5d133e
Reviewed-on: https://go-review.googlesource.com/32640

Reviewed-by: Quentin Smith <quentin@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>