image: Use unshare to map root user for btrfs

The current version of `mkfs.btrfs` has a regression when combined with
`fakeroot` where it does not detect the faked root uid/gid of files when
building the filesystem. This produces partition images with `/` owned by
the guixbuild user when it should be owned by root.

Using `unshare` rather than `fakeroot` resolves this by doing the uid
mapping at the kernel level rather than overloading the `stat` function.

An equivalent issue was [reported and patched in
NixOS](https://github.com/NixOS/nixpkgs/pull/434122)

* gnu/build/image.scm (system-disk-image): Import `util-linux` instead of
`fakeroot`.
* gnu/system/image.scm (make-btrfs-image): Use `unshare --map-root-user`

Change-Id: Id4eeaf510f3ec5f4a23b4d700a73e2cf46da40b1
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #5536

gnu/build/image.scm

@@ -83,7 +83,8 @@ turn doesn't take any constant overhead into account, force a 1-MiB minimum."
         (label (partition-label partition))
         (uuid (partition-uuid partition)))
     (apply invoke
-           `("fakeroot" "mkfs.btrfs" "-r" ,root
+           `("unshare" "--map-root-user" "mkfs.btrfs"
+             "-r" ,root
              "-L" ,label
              ,@(if uuid
                    `("-U" ,(uuid->string uuid))