mkarchiso: write a .uuid file to /.disk/ on ISO 9660 to allow GRUB to search for it

Create a `/.disk/INSERT_UUID_HERE.uuid` file to implement something
called "file system transposition".
See https://www.mail-archive.com/grub-devel@gnu.org/msg34346.html for
the proposal of it for grub-mkrescue.

This will allow GRUB to search for the volume containing the file instead
of hardcoding the ISO volume label or another file system or
partition attribute in GRUB configuration.
By only relying on the directory structure simplifies supporting medium
preparation methods with manual partitioning such as
https://wiki.archlinux.org/title/USB_flash_installation_medium#In_GNU/Linux_4

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