Seems like this should be more robust, and it will work even
after the activation script purifies its environment. Apparently
it may treat `tmux` sessions like SSH ones, which may have
something to do with `reattach-to-user-namespace`/`pam_reattach`; see
<https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard/issues/78#issuecomment-1074543831>.
(My hope is that `pam_reattach` does the right thing here, but I
haven’t tested.)
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