Commit 5395397f removed the assertions from the buildMachines to
ensure that either system or systems is set for each buildmachine.
This patch re-implements those assertions.
The symptom is that if both system and systems are omitted, then the
/etc/machines file has the wrong number of columns and any attempt to
run a `nix` operation that has to perform a build will fail with a
`strtoull` exception.
58921a4904