Docs are installed with wrong permissions

[brson]

The new installer has a bug that causes the docs to be installed in a way that can't be read by non-root.

[brson]

This is because the docs in-tree have non-world-readable permissions. I think we'll need a build step that just changes the permissions of everything in the doc directory.

[brson]

Actually, I'm going to fix this in rust-installer.

[brson]

Fixed in https://github.com/rust-lang/rust-packaging