Update and simplify the CI workflow

[cuviper]

• Update to actions/checkout@v3
• Switch from actions-rs/toolchain to dtolnay/rust-toolchain
• Switch from actions-rs/cargo to plain run
• Stop explicitly naming CI steps

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• Format
• No Std
• Test (1.15.0)
• Test (1.20.0)
• Test (1.26.0)
• Test (1.31.0)
• Test (1.8.0)
• Test (beta)
• Test (nightly)
• Test (stable)