`before_script` cannot start headless kakoune

Attempting to start a headless Kakoune session in a before_script causes tmuxp loading to hang forever. The Kakoune session is started correctly (so the script "completed" successfully), and an empty tmux session is started, so it looks like tmuxp is somehow waiting for before_script to complete even though it already has. This probably has something directly to do with the exact mechanism that Kakoune uses to "daemonize" itself, but nevertheless since running the command in a shell completes successfully, it should work as a before_script.

Steps to reproduce:

1. Have before_script: kak -s -d mykaksession in a config.
2. Run tmuxp load mytmuxp.yml

Observed Results:

• One Loading log line is printed, and then nothing happens ever.

Expected Results:

• The tmuxp session to load correctly.

Reproduce the issue with a tmuxp configuration containing before_script: kak -s -d mykaksession and tmuxp load mytmuxp.yml; then trace how before_script processes are started and awaited. Done means the Kakoune command can daemonize successfully without blocking, and the tmuxp session loads normally.