Silence tmux availability check

[4lllex]

When installed as a gem there is an unintended output from overmind command:

$ mise x ruby -- overmind -h | head -2
/usr/bin/tmux
NAME:

$ mise x go -- overmind -h | head -2
NAME:
   Overmind - The mind to rule processes of your development environment

The change doesn't affect the return value:

system("which tmux")
# /usr/bin/tmux
# => true

system("which tmux", out: IO::NULL)
# => true

system("which tshmux", out: IO::NULL)
# => false