Code review comment for lp://qastaging/~alan-griffiths/mir/fix-1290345-part2

Note to reviewers:

USC sets $MIR_SOCKET for itself in usc::ExternalSpinner::ensure_running() before execlp().

Unity8 doesn't (as far as I or Gerry knows) launch clients directly.

So I don't think setting $MIR_SOCKET in the server process is useful to anyone.