Ensure that the assets cache is readable.

Files created by mkstemp are "readable and writable only by the creating user ID."
The cache needs to be readable by the website user, even if it's created by root,
so change the mode on files added to it after they are created.
Fix error caused by previous commit.
Merge branch 'fix_unreadable_cache_orig' of github.com:aptivate/webassets