On Catalina, the /nix filesystem might not be mounted at start time.
To avoid this service not starting, we need to keep the launch agent
outside of the Nix store. A wait4pid will hold for our /nix dir to be
mounted.
Fixes#3125.
When using a volume, the nix-daemon path may not exist. To avoid this
issue, we must use the wait4path tool. This should solve one of the
issues in multi-user on macOS Catalina.
Since macOS 10.14 this has become an error, causing problems if the
nix-daemon loads nix during substitution (this is a forked process).
Workaround for #2523.