actions-rs/toolchain is outdated, use dtolnay/rust-toolchain instead

[sirandreww]

I notice that this example uses actions-rs/toolchain.
However, all of the most downloaded crates on crates.io use dtolnay/rust-toolchain.
While searching to find out why I found this:
https://www.reddit.com/r/rust/comments/vyx4oj/actionsrs_organization_became_unmaintained/
The PR linked there is correct. This tool chain is unmaintained, this example should be updated to match what the major crates are doing.