paths : fixes write_new_paths writing of the file

I was writing the file using 'w' instead of 'a' so the old paths were
deleted use 'a'.

src/unisync/paths.py

@@ -106,7 +106,7 @@ class PathsManager:
             if not is_contained and new_path not in paths_to_add:
                 paths_to_add.append(new_path)
 
-        with self.paths_file.open("w") as f:
+        with self.paths_file.open("a") as f:
             for p in paths_to_add:
                 f.write(p + "\n")