Export 'REPOS_DIR' when the selected source medium for package
installation is cdrom.  This enables bsdconfig(8) to make use
of the on-disc pkg(8) repository configuration, which fixes
package selection and installation from the dvd installer.

MFC after:	3 days
M-MFC-With:	r259426
X-MFC-Before:	-RC3
Sponsored by:	The FreeBSD Foundation