Add support for rust-toolchain.toml

[zephraph]

Fixes #208

This matches the behavior in rustup's docs namely:

the toolchain can be named in the project's directory in a file called rust-toolchain.toml or rust-toolchain. If both files are present in a directory, the latter is used for backwards compatibility.

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The previous implementation only supported a rust-toolchain file that included only the name. This is largely considered a legacy format at this point. I added fast-toml in order to be able to parse out the channel value. Technically channel is mutually exclusive with path that can also be specified to point to a file that contains the toolchain value somewhere else, but I've left that out of this implementation.

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If folks stumble across this PR and need this functionality you can use @oxidecomputer's fork at https://github.com/oxidecomputer/actions-rs_toolchain until this is merged.

[boozook]

Tested there, works 👍

[JasonShin]

Hi, it would be good to get this merged, is there anything that I can help to move it forward?

[mateimicu]

Hello,

I am also interested in this change. I can help if something is blocking it.

[smallstepman]

@mateimicu see https://github.com/actions-rs/toolchain/issues/216