oem: Enforce a specific mirror that will always be set

This is a hack, currently, which we need to ensure APT can fetch updates
in case we were unable to connect to the network while installing.
(if apt is not set up with sources, users will not know what to do at
all and assume the system is broken).

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

@@ -77,8 +77,10 @@ d-i finish-install/reboot_in_progress note
 
 #grub-pc ucf/changeprompt string install the package maintainer's version
 
-d-i preseed/late_command string cp /cdrom/purism/50_custom /target/etc/grub.d/50_custom;\
-  chmod a+x /target/etc/grub.d/50_custom;\
-  echo "GRUB_DISABLE_OS_PROBER=true" >> /target/etc/default/grub;\
+d-i preseed/late_command string cp /cdrom/purism/50_custom /target/etc/grub.d/50_custom; \
+  chmod a+x /target/etc/grub.d/50_custom; \
+  echo "GRUB_DISABLE_OS_PROBER=true" >> /target/etc/default/grub; \
+  echo -e "#\n\ndeb http://repo.puri.sm/pureos/ green main" > /target/etc/apt/sources.list; \
+
 in-target sed -e '/boot/s/^#*/#/' -i /etc/fstab;\
 in-target update-grub;