ebuild.sh: fix pkg_setup env for binary packages (528274)

In commit 743b3337644fbb3ea461d3a9137c9ac85e163f03, support for binary
packages broke, since binary packages must load the environment from
$T/environment. This patch fixes the logic to handle binary packages
correctly.

Fixes: 743b3337644f ("ebuild.sh: force fresh env for pkg_setup (528274)")
X-Gentoo-Bug: 528274
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=528274