ci: Use explicit clippy version

[caspermeijn]

Using an explicit version of clippy will to prevent CI breaking when new lints are introduced.

[tomhoule]

Hi @caspermeijn , thanks for the PR. I'm going to take on slightly more active maintainership on this crate again. I'm split on this — on the one hand, it avoids PRs having to deal with unrelated lint errors after an update. On the other hand, we do want the new lints. I think it's an ok tradeoff to manually update this variable when we want the new lints though, let's do this.

[caspermeijn]

I agree updating the clippy version is a burden. But I think it is worth it, as a PR opened in one year which fails due to a new clippy lint can be demotivating for a new contributor.