'WindowsError: [Error 32]' from renaming the downloaded file while it's still open.

The following occurs on Windows with the --persist option.

  File "build\bdist.win32\egg\mozregression\runnightly.py", line 308, in cli
  File "build\bdist.win32\egg\mozregression\runnightly.py", line 249, in start
  File "build\bdist.win32\egg\mozregression\runnightly.py", line 242, in install

  File "build\bdist.win32\egg\mozregression\runnightly.py", line 89, in download

  File "build\bdist.win32\egg\mozregression\utils.py", line 58, in download_url
WindowsError: [Error 32] The process cannot access the file because it is being
used by another process