SystemTools: Do not free buffer passed to putenv("A=") on Windows

When using `putenv("A=")` to approximate removal of `A` from the
environment we must not free the buffer afterward because the string
is still actually in the environment.

Change-Id: I4d2455ca604a2e216123a3d2afafb64ca260d105
Reported-by: James Johnston <johnstonj.public@codenest.com>