images/server: Rename package_selection for custom-repo

With package_selection set to `custom` for custom-repo 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 could rename package_selection to `custom-repo-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

@@ -126,7 +126,7 @@ case "${install_packages_from}" in
     ;;
     custom-repo)
         get_custom_repo "${install_custom_repo}"
-        package_selection=${package_selection:-custom}
+        package_selection=${package_selection:-custom-repo-devbuilds}
     ;;
     custom-devbuilds)
         get_custom_repo "${install_custom_repo}"