add desk_sync to .local/bin, change default flake in makefile to be the cwd, change desk_sync to use sh instead of bash

dotfiles/local/bin/desk_sync

@@ -1,3 +1,3 @@
-#!/bin/bash
+#!/bin/sh
 unison /home/tom/Desktop ssh://tom@tmoron.fr:1880//home/tom/Desktop_sync/Desktop $@