message: buildenv: Don't err if build tree already exists.
diff: @@ -19,7 +19,9 @@ def json_converter(o):
  raise TypeError('%s is not JSON serializable' % o)
  def copy(f, t):
  - os.makedirs(os.path.dirname(t))
  + parent = os.path.dirname(t)
  + if not os.path.exists(parent):
  + os.makedirs(parent)
  sh.cp('-a', f, t)
  def mv(f, t):
  