TPM causes my AppImage build to hang on start if no server is running #189

Steps to reproduce:

1. Grab my AppImage build here https://github.com/michaellee8/tmux/releases/tag/test or build a flash one with ./tools/genappimage.sh if you don't trust me. It is the only file I have added in my fork, it requires pkg-config, autotools, automake, autoconf and imagemagick.
2. Make sure there are no tmux server running (kill all running tmux process)
3. Make sure you have the run tpm entry on your .tmux.conf
4. ./tmux.appimage -vv (-vv to get server log)
5. Get hang and not even Crtl+C can help you.
6. Kill all tmux process.
7. Comment out the run tpm line on your .tmux.conf
8. Let's run ./tmux.appimage again, nicely running.
9. Look at the bottom of the log you can see some TMUX_PLUGIN_MANAGER_PATH not found error.

• Only breaks if there are no server running. If you let that server running without closing the first client and statrt the second client, the second client works fine.
• Works fine with the system's tmux, I don't know whether it is because it is old enough to not have the bug or because the system tmux don't need a TMUX_PLUGIN_MANAGER_PATH.