main/tmux: fix server crashes in sixel terminals
neovim 0.10.0 crashes the tmux server when run as a different user than the one that started the tmux server (https://github.com/tmux/tmux/issues/3983).
It is fixed by this commit (https://github.com/tmux/tmux/commit/aa17f0e0c1c8b3f1d6fc8617613c74f07de66fae), which I copied as a patch in this MR.