FLUX_TERMINUS_SESSION is always set in the job environment

[garlick]

Problem: As noted in #5527, FLUX_TERMINUS_SESSION seems to be set in the job environment even when a pty was not requested.

$ flux run  printenv FLUX_TERMINUS_SESSION
0

[grondo]

I had to look back at the implementation, but the purpose of FLUX_TERMINUS_SESSION is to prevent "nesting" pty sesssions, since the "commands" sent to the server can't be relayed to the deepest nested session. I have to think (and experiment) if the FLUX_TERMINUS_SESSION should or should not be set for tasks without a pty, since there is a terminus "server" started for every job (which kind of acts like a tmux server and would techincally let you "attach" an interactive shell to any job)