prepare: Fix ownership of Portage DISTDIR

If the Portage DISTDIR is a mounted filesystem, it may not have the
correct permissions initially.

prepare.sh

@@ -10,6 +10,8 @@ if [ ! -f /var/db/repos/gentoo/metadata/timestamp ]; then
     emerge-webrsync
 fi
 
+chown portage: "$(portageq envvar DISTDIR)"
+
 ARCH=$(PORTAGE_CONFIGROOT=/usr/${target} portageq envvar ARCH)