cmd/go: skip tests using Git on Plan 9

TestScript/get_unicode, TestScript/get_dotfiles and
TestScript/get_brace are failing on Plan 9 since they
expect a full-featured git command, while the git tool
has been emulated as a simple rc script on Plan 9.

This change skips tests using Git on Plan 9.

Fixes #29640.

Change-Id: Id7f6fdca552167f4631fe401f63167e5653daafa
Reviewed-on: https://go-review.googlesource.com/c/157119


Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>