sway: fix sway-launcher

whence is a zsh builtin, use bash's compgen function to get a list
of available commands

pkgs/sway-launcher.nix

@@ -11,7 +11,7 @@ with self; ''
 
   # Get shell command list
   # This may include the occasional non-executable file
-  command_list=$({ whence -wm '*' | sed 's/:[^:]*$//' })
+  command_list=$({ compgen -c | sed 's/:[^:]*$//'; })
 
   # read existing command history
   if [ -f "$HIST_FILE" ]; then