system: Add store-directory-prefix to boot-parameters.

system: Add store-directory-prefix to boot-parameters.

Fixes <http://issues.guix.gnu.org/44196>

* gnu/machine/ssh.scm (roll-back-managed-host): Use
boot-parameters-store-directory-prefix.
* gnu/system.scm (define-module): Export
boot-parameters-store-directory-prefix.
(<boot-parameters>)[store-directory-prefix]: New field. It is used to
generate the correct paths when /gnu/store is installed on a btrfs
subvolume whose name doesn't match the final runtime path, as the
bootloader doesn't have knowledge about the final mounting points.
[boot-parameters-store-directory-prefix]: New accessor.
(read-boot-parameters): Read directory-prefix from store field.
(operating-system-boot-parameters-file): Add directory-prefix to
store field.
* guix/scripts/system.scm (reinstall-bootloader): Use
boot-parameters-store-directory-prefix.
* test/boot-parameters.scm (%default-btrfs-subvolume,
%default-store-directory-prefix): New variables.
(%grub-boot-parameters): Use %default-store-directory-prefix.
(%default-operating-system): Use %default-btrfs-subvolume.
(test-boot-parameters): Add directory-prefix.
(test optional fields): Add test for directory-prefix.
(test os store-directory-prefix): New test.