oem: Don't auto-configure an APT mirror

We set it statically, so the user isn't confused with weird messages if
no internet connection is established.

config/includes.chroot.oem/var/lib/pureos-oem/di-preseed.cfg

@@ -9,7 +9,11 @@ d-i apt-setup/cdrom/set-first boolean true
 d-i apt-setup/cdrom/set-double boolean false
 d-i apt-setup/cdrom/set-next boolean false
 d-i apt-setup/cdrom/set-failed boolean false
-d-i apt-setup/use_mirror boolean true
+
+# we won't have an internet connection, so we disable
+# mirror selection.
+# A mirror will be set by the late_command preseed at the end
+d-i apt-setup/use_mirror boolean false
 
 d-i mirror/country string manual
 d-i mirror/http/hostname string repo.puri.sm