How to offload builds only when some of the offload build servers are available

[Kyle Andrews]

Dear Guix,

How can I configure the offload functionality to check whether the build
machines specifiec in ~/etc/guix/machines.scm~ are available first; and
if they are not (e.g. because I am roaming outside my local network)
just proceed with builds locally so that I don't have to remember to
pass --no-offload?

Thanks,
Kyle