Shim has had a hard time with loading updates from arguments in the
most recent release and this isn't the first time that happened.
Give distros and users an escape hatch that will allow using GRUB
instead.
The systemd shutdown script gets called after /snap/fwupd/* gets
unmounted meaning it can't be used to do the activation.
Explicitly check that the symlink for /snap/fwupd/current is mounted
when calling the script.
