offload: Recognize build failures due to lack of disk space.

Previously, if a remote build would fail due to lack of disk space, this
would be considered a permanent failure and thus cached as a build
failure if the local daemon runs with '--cache-failures'.

* guix/scripts/offload.scm (transfer-and-offload): Upon
'nix-protocol-error?' call 'node-free-disk-space' and return 1 instead
of 100 if the result if lower than 10 MiB.