Don't override PATH while installing requirements.txt

Setting the path here causes installation failures if some package
in requirements.txt needs tools which happen to be installed in
a non-system location.

Leave path alone so caller can use Exec { path=>[...] } or similar
to locate the desired tools.

Signed-off-by: Ray Lehtiniemi <rayl@mail.com>
Merge pull request #97 from rayl/master

Don't override PATH while installing requirements.txt