nixos/lib/testing: Fix unsetting $xchg

Regression introduced by 4dcb685a

.

Unsetting the environment variable shortly before using it is not going
to end up very well, so let's just filter out the variable from the
output of export and unset it shortly afterwards.

This fixes the runInMachine NixOS test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>