feat(install.d): add sort-key field to rescue BLS entries

The `sort-key` field was added in systemd/systemd#22519
and is used by systemd-boot to sort boot entries. Without it, rescue entries
generated by this plugin are not sorted properly.

This patch mimics systemd/systemd@d23b3bfd

install.d/51-dracut-rescue.install

@@ -38,6 +38,9 @@ fi
 
 [[ -n $PRETTY_NAME ]] || PRETTY_NAME="Linux $KERNEL_VERSION"
 
+SORT_KEY="$IMAGE_ID"
+[[ -z $SORT_KEY ]] && SORT_KEY="$ID"
+
 if [[ ${KERNEL_INSTALL_MACHINE_ID+x} ]]; then
  MACHINE_ID=$KERNEL_INSTALL_MACHINE_ID
 elif [[ -f /etc/machine-id ]]; then
@@ -127,6 +130,7 @@ case "$COMMAND" in
  echo "title $PRETTY_NAME - Rescue Image"
  echo "version $KERNEL_VERSION"
  echo "machine-id $MACHINE_ID"
+ [[ -n $SORT_KEY ]] && echo "sort-key $SORT_KEY"
  echo "options ${BOOT_OPTIONS[*]} rd.auto=1"
  echo "linux $BOOT_DIR/linux"
  echo "initrd $BOOT_DIR/initrd"