offload: Skip machines that are low on disk space.

Fixes <https://bugs.gnu.org/33378>.

* guix/scripts/offload.scm (node-free-disk-space): New procedure.
(%minimum-disk-space): New variable.
(choose-build-machine): Call 'node-free-disk-space' and take it into
account in addition to LOAD.
(check-machine-status): Display the free disk space.