Update and simplify the CI workflow

[cuviper]

• Update to actions/checkout@v3
• Switch from actions-rs/toolchain to dtolnay/rust-toolchain

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• tests (1.36.0)
• tests (beta)
• tests (nightly)
• tests (nightly, i686-unknown-linux-gnu)
• tests (stable)
• tests (stable, mips64-unknown-linux-gnuabi64)