bin/ebuild-helpers/portageq: fix bug #524964

Since commit 0cc4c1ac21a2ea94cfb1f6ff4b461a9e349d47df,
$PORTAGE_BIN_PATH/portageq no longer exists, which breaks
bin/ebuild-helpers/portageq. Note that has_version and best_version
rely on bin/ebuild-helpers/portageq if IPC is disabled, so breakage
extends beyond ebuilds that call portageq "illegally". Since
$PORTAGE_BIN_PATH no longer works, use PATH to locate the real
portageq python script.

Fixes: 0cc4c1ac21a2 ("Install Portage using setup.py")
X-Gento-Bug: 524964
X-Gentoo-URL: https://bugs.gentoo.org/show_bug.cgi?id=524964