Spices.py: Preserve original file permissions of downloaded spices

[fredcw]

Preserve original file permissions of all files in downloaded spice zipfile. This is an improvement on commit https://github.com/linuxmint/cinnamon/commit/42b1e53ef4ae0d26df7a8e8f588cdcfd47be2b12 which give all files the same permissions.

[anaximeno]

I agree, I was thinking about this recently too, since it could be a vulnerability point.