revision 1.12 of file_util.py

[Bug #220993; may also fix bug #479469] Fix flakiness when old
  installations are present, by always unlinking the destination file
  before copying to it.  Without the unlink(), the copied file remains
  owned by its previous UID, causing the subsequent chmod() to fail.

Bugfix candidate, though it may cause changes on platforms where
  file ownership behaves differently.