chore: Fix CI install rust toolchain

[jamesmunns]

Also, incidentally, i think the current "install rust toolchain" step is wrong everywhere, since it's running before checkout and doesn't include the toolchain file. it will always show the default toolchain on the github actions runner, and not the toolchain in our rust-toolchain.toml...so it's basically pointless. And, furthermore, I think the recent Rustup changes to stop implicitly installing toolchains (rust-lang/rustup#3635) would have broken this if they weren't reverted (rust-lang/rustup#4211), since rustup show would no longer install the required toolchain if it's not present. So, we should probably change all the "install rust toolchain" steps by:

• moving them after the checkout step, and
• making them do rustup toolchain install rather than rustup show.

But, this should be done in a separate PR.

Originally posted by @hawkw in #526 (comment)