config.environ(): filter PORTDIR only when required (bug 653230)

The changes related bug 373349 were more aggressive than necessary,
causing the PORTDIR variable to be omitted from config.environ()
calls when the EAPI is defined. We really only need to filter the
PORTDIR variable when the current EAPI requires it.

Fixes: 802e7d0bdd96 ("Do not export PORTDIR & ECLASSDIR in EAPI 7")
Bug: https://bugs.gentoo.org/653230
1 file changed