While starting the tmux sessions from fish worked some environment
variables were not getting picked up in tmux session.
For example DISPLAY is set after we log in and the graphical sway
session starts but since tmux sessions were started before that it
would be absent from the tmux session. We may be wrong about the
exact order and reasoning for this but atleast that is the observed
behaviour.
Fix this by defining a sway session target and making the tmux session
service dependent on it.
