fix: Gracefully handle read-only sdists

For a package without dependencies in pypi's database, like p4python,
the sdist is required.  The problem is p4python was developed in
perforce where all files are read-only by default and deleting the temp
directory fails.

So we need to use the custom-built temp directory and specially handle
read-only files to be able to use p4python in poetry.

Fixes #520