images/server: Rename package_selection for custom-repos

With package_selection set to `custom` for custom-repos source we
failed to install libcephfs-proxy2, samba-vfs-cephfs and few other
required packages due to non satisfying switch case down the line.
Instead we rename package_selection to `custom-repos-devbuilds` so
as to enter the relevant switch case to install crucial packages.

Signed-off-by: Anoop C S <anoopcs@samba.org>

Author: anoopcs9

images/server/install-packages.sh

@@ -131,7 +131,7 @@ case "${install_packages_from}" in
     ;;
     custom-repos)
         get_custom_repos "${install_custom_repos}"
-        package_selection=${package_selection:-custom}
+        package_selection=${package_selection:-custom-repos-devbuilds}
     ;;
     custom-devbuilds)
         get_custom_repos "${install_custom_repos}"