Remove use of deprecated actions-rs/cargo GH action

[rgwood]

Our CI actions have a lot of warnings like this:

nu-fmt-clippy (ubuntu-20.04, stable)
Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions-rs/[email protected]

It looks like actions-rs/cargo is abandoned. But the good news is we don't actually need it, we can just run cargo subcommands without a special action because we've already installed cargo with actions-rust-lang/setup-rust-toolchain.

[sholderbach]

Much nicer. Thanks!