test-env-util: allow $PATH to be unset

Coverity was unhappy, because it doesn't know that $PATH is pretty much always
set. But let's not assume that in the test. CID #1400496.

$ (unset PATH; build/test-env-util)
[1]    31658 segmentation fault (core dumped)  ( unset PATH; build/test-env-util; )