Accept different tmux installation path in systemd unit file

[primeapple]

When creating the systemd unit file on linux, we don't check where the tmux installation actually is, we just assume it is on /usr/bin/tmux. I'm using asdf to install tmux, so the executable is somewhere else. Because I don't like to symlink my executables to /usr/bin/, I created this fix to check the actual path of the executable when creating the systemd file.

Tested and works. :)

[bruno-]

Thanks