Ensure selfcheck file inherits directory permissions

[amol-]

When writing files to cache pip does copy the permissions from the cache directory, this is an expected behaviour to ensure that any user that has access to the cache has also access to its content.

Copying the permissions explicitly is necessary due to the use of adjacent_tmp_file which by virtue of tempfile.mkstemp (indirectly via NamedTemporaryFile) causes all files to be created with 600 permissions.

This is not happening for selfcheck/XXXXX by the way, which makes the selfcheck unaccessible by users different from the one that created the cache.

This PullRequest creates an helper method copy_directory_permissions which allows both the cache and the selfcheck file to share the same logic in copying directory permissions and thus ensure consistency in cache files permissions.

[notatallshaw]

Hi @amol- thanks for your PR to pip. Please be aware it take the maintainers some time to review as we are all doing this on a volunteer basis.