This should make the test more robust, considering the strange hang
in https://hydra.nixos.org/build/267517233/nixlog/8
`builder` seems to have reached `multi-user.target` before the
SSH connection was established, but this seems to be coincidental.
This does tell us that enforcing this has a minimal cost in terms
of runtime.
Waiting for `multi-user.target` on the client is honestly paranoid,
but flaky tests are very bad for productivity.
7c5a0b06a4