update.d/efi_sync_flag: Fix missing parameter in parted_server command

The missing paramater makes the installer user interface hang when the parted_server command is executed.

Currently the command is executed under specific conditions (e.g. create a EFI partition and commit it or use an existing FAT32 partition as EFI, then use it as something else or do not use it) due to subtle interaction with the 'boot' flag management in partman-base. I triggered the bug while testing a patch making partman-efi stop improperly using the 'boot' flag (and use only the 'esp' flag as it should).

Closes: #1085849