fix: envvar_value logic to handle '=' in tmux variable values

I wanted to embed styles in `FLOAX_TITLE`, but the current `envvar_value` function couldn't correctly extract the value when it contains an equal sign `=`. This fix addresses that issue. ```bash tmux setenv -g FLOAX_TITLE "[#align=centre,fg=red]My Test Title" tmux showenv -g "FLOAX_TITLE" | cut -d '=' -f 2 # [#align tmux showenv -g "FLOAX_TITLE" | cut -d '=' -f 2- # [#align=centre,fg=red]My Test Title ```