[Nix-dev] Accessing systemPackages from postStart

[Taeer Bar-Yam]

General problem:
  How do I access command line tools (that are in systemPackages) from some systemd.services.<name>.postStart?
Specific problem:
  I want that when mopidy starts up, use mpc to select a playlist and turn on shuffle. However, I am getting an `mpc: command not found` error. I can access mpc normally, just not from this startup script (declared as systemd.services.mopidy.postStart = "<call mpc here>")

  --Taeer