syscall: append to environment in tests, don't clobber it

syscall: append to environment in tests, don't clobber it

This is a partial backport of https://golang.org/cl/233318.
It's only a partial backport because part of the change was
already applied to libgo in CL 193497 as part of the update
to the Go 1.13beta1 release.

Fixes PR go/95061

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/233359