Overwriting file-cached keys on Windows results in an access denied error

This is most easily reproduced by running PhpBackendUnitTest, which fails on Windows resulting in a error similar to:

rename(sites/simpletest/454911/files/php/cache_page/.2182f2ee82,sites/simpletest/454911/files/php/cache_page/4DdMbToh62rmlatZJ9JZhKIsaANqRJmDb53GpRYrTfI/77b96f84dee112a2bc5cbb6c09722bee07bde3bd8bc78675274980ca21cf3378.php): Access is denied. (code: 5)

This is because rename() on Windows incorrectly uses the target file permissions rather than the containing directory permissions. See this PHP bug report.