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 <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>