Test talking to an older daemon

[thufschmitt]

Is your feature request related to a problem? Please describe.

There's no way atm to test that Nix master will behave correctly if the daemon is older (or vice-versa, but that's probably less an issue). Yet a failure due to a bad interaction between different versions pops-up every so often.

Describe the solution you'd like

Add a test (at least a CI job if it's too complicated to do that locally) that runs (a subset of) the testsuite using older daemons (at least the latest released version, dunno if it's worth adding more).