configs/*/grub/grub.cfg: search for a .uuid file in /.disk/ and use the volume it's on

Search for `/.disk/%UUID_SEARCH_FILENAME%.uuid` and pass the UUID of the
volume it's on as `archisodevice`. mkarchiso will replace
`%UUID_SEARCH_FILENAME%` with a hardcoded value generated using
`SOURCE_DATE_EPOCH` durring ISO build.

This allows to prepare an UEFI bootable installation medium by simply
copying the directory structure without having to touch `grub.cfg`.

Relying on the volume UUID instead of its LABEL also avoids collisions
of multiple ISOs created in the same month.

Fixes #202

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