bin/ebuild: allow useful phases with pkg_config

It makes sense to run pkg_config by itself, but special phases like
"clean", "digest", and "manifest" can be useful. For example, if the
developer modifies the ebuild, then it's useful to regenerate the
manifest and use "clean" to discard the stale environment.

Fixes: bfa98d7a5b7a ("fixed ebuild so")
Reported-by: Vlastimil Babka <caster@gentoo.org>
Reviewed-by: Alexander Berntsen <bernalex@gentoo.org>