Use rust-toolchain.toml for developers and CI

[rukai]

The changes are:

• setup a rust-toolchain file using the modern format: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
• removed toolchain specification from github actions config so that it will fall back to the toolchain file: https://github.com/actions-rs/toolchain

I hope my CI changes do what I think they should... I cant really test them locally.

Dependent on: #166

This change could be controversial so I'm happy to discuss it.
The advantages I see here are:

• CI doesnt break due to nightly regressions or breaking changes.
• Developers work on a common nightly, preventing differences between nightlies from causing hard to track down issues.
• Users have a documented nightly version that embassy supports, and they can also copy in the embassy rust-toolchain file without having to setup their own.

A disadvantage is:

• If a user wants to try building one of the examples it will pull in rustfmt and a target they dont need.

However I dont think that is a big problem because if they are bothered by that they can just move the example into its own crate. Which they will probably want to do eventually anyway if they have interest in using embassy.

[thalesfragoso]

As a note, I would choose a toolchain with clippy available :)

[rukai]

Did some research and found this: https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu.html
I dont know if we care about about miri and rls but I figured I may as well support them by downgrading by one day. So I downgraded to 2021-05-07.

I also fixed the CI build by telling actions-rs/toolchain to use the stable toolchain (which is then superseded by the rust-toolchains.toml)

[Dirbaio]

Thanks for the PR! I like this :)

Pinning a particular nightly version is very nice, it'll hopefully help with the random CI failures and allows updating to newer nightlies at our pace