Support toolchain toml

[boozook]

Currently we're using fork by @zephraph with patch for the subject, since #176.

We have to switch back to using original actions-rs/toolchain when that PR and issue closed.
Just replace zephraph/toolchain@support-toolchain-toml to actions-rs/toolchain@v1 anywhere.

Checklist:

• tests
• release
• coverage

[zephraph]

Instead of using my fork I'd recommend using https://github.com/oxidecomputer/actions-rs_toolchain. I migrated it to @oxidecomputer's org because we have a vested interest in Rust and its infrastructure and there's more eyes there beyond just my own. If other changes come up that need to be added, that's where it'll happen. I will, of course, continue to work to try to get these updates merged upstream.

[boozook]

@zephraph thanks! Done.