Update CI

[waywardmonkeys]

• Update to actions/checkout v4 from v2.
• Switch to dtolnay/rust-toolchain. The old actions-rs actions haven't been maintained in years.
• Stop using xargo for no_std builds and use cargo instead. Also, use better targets that have no std library available for them.

[waywardmonkeys]

@sebcrozet This should fix the CI builds for this crate (and some other stuff).