Investigate using sccache with local disc cache on CI

[matklad]

Binary release of sccache can be downloaded from https://github.com/mozilla/sccache/releases

See: rust-lang/cargo#5885 (comment)

We still spend a lot of time on CI when uploading caches.

Perhaps sccache will give us a more stable compilation cache?

[matklad]

This before_cache dose the trick:

https://github.com/rust-analyzer/rust-analyzer/blob/4a3d6aa26a1c71dadc91f37f9d5f8662b51c138a/.travis.yml#L2-L5