compress/gzip: skip builder-only test on iOS
compress/gzip: skip builder-only test on iOS

The new iOS test harness forwards environment variables to the
test program, which means that it runs builder-only tests that were
previously skipped because GO_BUILDER_NAME was missing.

Skip one such unblocked test, TestGZIPFilesHaveZeroMTimes, which
assumes a valid GOROOT.

Change-Id: I5daf0f4c1897afbeb8b1a380669a1d2aa47e764a
Reviewed-on: https://go-review.googlesource.com/111475
Run-TryBot: Elias Naur <993af7099cb24ce1a88d64108ae1e8b0385f2fc0@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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