trigger.py: Remove GTest shard env when running one shard. Bug: angleproject:5158 Change-Id: I3c29adf36f6f31f23bf662c2d7d6c4d05d8e9028 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2466781 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>