Change Rust toolchain to 1.37.0

[lopopolo]

Artichoke does not require nightly and recently updated its toolchain to 1.37.0 artichoke/artichoke#177

Build the playground with the same toolchain.

Also pin the CircleCI Rust container to buster-1.37.0 artichoke/artichoke#190

[lopopolo]

Fixed in GH-23.