chore: fix last clippy lints and enforce in CI

[nyurik]

• turns out clippy was part of CI, but was not used to enforce anything
• clippy should not use nightly channel
• check-cfg has been stabilized since 1.80, so no need for a separate CI job
• rustfmt is the only job requiring nightly

Also, I think we should remove installation of the stable rust - github runners have been very good at keeping rust version up to date, no need to waste time for each job to install it.