cmd/go: skip gitrepo tests on Plan 9
cmd/go: skip gitrepo tests on Plan 9

CL 118095 added gitrepo tests. These tests 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.

Fixes #25938.

Change-Id: I262a89d0ce83168c550d9af3e832ed3a1e3c43f6
Reviewed-on: https://go-review.googlesource.com/122455
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,CAFxX/go,akutz/go,josharian/go,akutz/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,golang/go,akutz/go,golang/go,cloudflare/go,josharian/go,golang/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,josharian/go,golang/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,josharian/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,akutz/go,josharian/go,akutz/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go