bin/ebuild-helpers/doins: fix PYTHONPATH setting (bug 640934)

This makes the PYTHONPATH setting consistent with other helpers
like dohtml and xattr/install.

Fixes: d9522ba661b5 ("Rewrite doins in python (bug 624526)")
Bug: https://bugs.gentoo.org/640934
(cherry picked from commit 7ae0c9d084bb1caf18c751cecb7fb98a619dfbcb)

BUG=chromium:712659
TEST=precq passes

Change-Id: Ie7c5520665e3be9c6d90f68a2614096ec75814a3
Reviewed-on: https://chromium-review.googlesource.com/831071
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
diff --git a/bin/ebuild-helpers/doins b/bin/ebuild-helpers/doins
index 28f9f4d..cfae3dd 100755
--- a/bin/ebuild-helpers/doins
+++ b/bin/ebuild-helpers/doins
@@ -82,7 +82,7 @@
 # If PYTHONPATH is empty (not unset), it means "add current working directory
 # to the import path" if the Python is prior to 3.4, which would cause
 # unexpected import. See also #469338.
-PYTHONPATH="${PORTAGE_PYM_PATH:-/usr/lib/portage/pym}${PYTHONPATH:+:}${PYTHONPATH}" \
+PYTHONPATH=${PORTAGE_PYTHONPATH:-${PORTAGE_PYM_PATH}} \
 	"${PORTAGE_PYTHON:-/usr/bin/python}" \
 	"${PORTAGE_BIN_PATH:-/usr/lib/portage/bin}"/doins.py \
 	"${DOINS_ARGS[@]}" -- "$@" || \