Add ability to pin rustup version

[sffc]

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• Is this related to the actions-rs Actions?
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Motivation

Generally, the rustup tool is stable across releases. However, the 1.25.0 release broke our GitHub Actions CI (rust-lang/rustup#3029). Since this is CI, any failures get in the critical path of our development workflow, so it's crucial to be able to roll out a fix as soon as possible. Pinning the rustup version to 1.24.3 should have been able to fix this problem, but there does not appear to be an easy way to do this in actions-rs.

Workflow example

      - uses: actions-rs/toolchain@v1
        with:
          toolchain: stable
          rustup-version: 1.24.3

Additional context

I think the change would go here:

core/src/commands/rustup.ts

Line 50 in 4487ec1

public static async install(): Promise<RustUp> {

Currently it always installs from a fixed path:

const rustupSh = await tc.downloadTool('https://sh.rustup.rs');

Instead, we would need to install from a path such as:

https://static.rust-lang.org/rustup/archive/1.24.3/x86_64-unknown-linux-gnu/rustup-init

Additionally, I am unsure of exactly how this works when there are multiple actions-rs steps in the same workflow. If one of them has the rustup version specified, and another has a different or missing rustup version, how do we determine which rustup version to install?

CC @Manishearth @aethanyc @robertbastian