The garbage collector no longer blocks other processes from
adding/building store paths or adding GC roots. To prevent the
collector from deleting store paths just added by another process,
processes need to connect to the garbage collector via a Unix domain
socket to register new temporary roots.
When `NIX_DAEMON_PACKAGE` is set, make all the tests use the Nix daemon.
That way we can test every piece of Nix functionality both with and
without the daemon.
Tests for which using the daemon isn’t possible or doesn’t make sens can
selectively be disabled with `needLocalStore`
