bug#44196: Problems with /gnu/store in a different btrfs subvolume

["Miguel Ángel Arruga Vivas" <rosen644835@gmail.com>]

I've been testing the installation and the use case for separate btrfs
subvolumes, so I created two different btrfs subvolumes for the root
file system (/rootfs) and the store (/storefs), and installed with guix
system init and a basic operating-system configuration.

The problems detected were:

  - [ ] Grub localization doesn't properly work as the root file system
        is not located in the literal route (that I hard-coded based on
        grub defaults).
        This was the main test, I still don't have a patch, but I'm
        thinking that we should generate the locale and provide there
        the store path.  WDYT?
  - [*] The keymap doesn't work on stage2, as the path contains the
        wrong prefix for the store file.  Fixed with patch 1.
  - [?] The store-prefix was not being provided in other places than
        the generation of a new system generation (sic), so
        "guix system delete-generations" generates grub.cfg with wrong
        paths.  This should be fixed with patch 2, but I'm not sure how
        to write another test with a marionette: gnu/tests/installer.scm
        contains most of the code needed, but I'm not sure how to do it.

The patches will follow this email.