ebuild.sh: force fresh env for pkg_setup (528274)

In ebuild.sh, $T/environment is loaded for most phases, but it should
always start with a fresh environment for pkg_setup. Therefore, skip
environment loading for pkg_setup, and force the ebuild to be sourced.

This also requires a change in the config.environ method, in order
to ensure that the calling environment is inherited by pkg_setup
(even though $T/environment may exist).

X-Gentoo-Bug: 528274
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=528274