David Tolnay cd836eb3ca Try caching Cargo index to speed up 1.13 build ...

This one build always took several (10+) minutes on:

    Updating registry `https://github.com/rust-lang/crates.io-index`

2020-05-03 17:59:26 -07:00

..

ci.yml

Try caching Cargo index to speed up 1.13 build

2020-05-03 17:59:26 -07:00