Fix setup-rust action in github actions

[craigds]

Description

Uses rustup to install rust in the Linux build, rather than the recently-deleted ATiltedTree/setup-rust action.

The MacOS build uses dtolnay/rust-toolchain@stable - I tried to use this, but the ancient manylinux2014 image doesn't have a new-enough curl version, so that action dies. So I just curled it manually; seems to work

Related links:

Checklist:

• [x] Have you reviewed your own change?
• [ ] Have you included test(s)?
• [ ] Have you updated the changelog?