doebuild: export EPREFIX to the environment as is

doebuild: export EPREFIX to the environment as is

Stripping a trailing slash at this point is kind of pointless, since
EPREFIX is used all over the code base without this stripping.  Either
configure, or portage.const_autotool should ensure it doesn't end with a
slash.  Removing slash stripping here, incidently helps people who use
Prefix Portage without an offset, and as hack use '/' as offset there.