phase-helpers.sh: has/best_version -b in any phase (bug 665038)

Since BROOT is only defined in src_* phases, make has/best_version -b
use the equivalent /${PORTAGE_OVERRIDE_EPREFIX#/} value that is defined
in all phases. This makes has/best_version -b in EAPI 7 behave exactly
the same as --host-root behaves in EAPI 6, allowing python ebuilds to
call python_setup in any ebuild phase.

Bug: https://bugs.gentoo.org/665038
Reviewed-by: Brian Dolbec <dolsen@gentoo.org>
1 file changed