tmux: shell through env

dot_tmux.conf

@@ -35,4 +35,8 @@ bind '"' split-window -v -c "#{pane_current_path}"
 if-shell '[ -x /usr/bin/fish ]' {
     set -g default-shell /usr/bin/fish
 }
+# use custom shell if configured with ENV
+if-shell '[ -x $TMUX_SHELL ]' {
+    set -g default-shell $TMUX_SHELL
+}