Adjust GH Actions

[dhardy]

Tweak the GH Actions:

• Clippy and rustfmt both now run on rustc stable. (We did pin Clippy to 1.78, but no one remembered to update this. Avoiding usage of Clippy on nightly does avoid many of its update problems in my experience.)
• Run Clippy and rustfmt from the main test job, not a dedicated job. This is easier to read on the Actions page in my opinion.
• Run benchmark tests from the benches job. This is more logical. It does mean we need to run that job on changes to the main source tree, but that's not a big deal IMO since the Clippy/rustfmt jobs are very fast.
• Likewise for distr_test
• Tweak rules on when jobs run

[newpavlov]

Clippy and rustfmt both now run on rustc stable.

Clippy routinely gets new lints. So using the stable toolchain can result in unrelated CI failures on new stable releases. This is why I think it's worth to keep a fixed version and bump it manually from time to time.

[dhardy]

I know; I've been hit by a few that appeared on nightly (often being adjusted before hitting stable). But I suspect we will forget to update this otherwise, and we would need to specify in three places.