Previously, `os.Clearenv()` (by way of `syscall.Clearenv`) would simply
set all environment variables' values to `""` rather than actually
unsetting them causing subsequent `os.LookupEnv` calls to return that
they were still set.

Fixes #17902

Change-Id: I54081b4b98665e9a39f55ea7582c8d40bb8a2a22
Reviewed-on: https://go-review.googlesource.com/33168
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>