Windows process cannot run properly, if it only has
GOOS and GOARCH environment variables set. It needs
other environment variables. So adjust TestLarge and
TestNoRet to add GOOS and GOARCH to the existing
variables set instead of clearing environment.

Fixes #24855

Change-Id: I9fd9430d89031c3bacdbc6283450efaa4819e616
Reviewed-on: https://go-review.googlesource.com/107035
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>