ci: Pin clippy to rust 1.78.0

[ValuedMammal]

This PR pins clippy check in CI to the rust 1.78 toolchain, which prevents new lints in stable releases from interrupting the usual workflow. Because rust versions are released on a predictable schedule, we can revisit this setting in the future as needed (say 3-6 months).

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [ ] I ran cargo fmt and cargo clippy before committing

[apoelstra]

In rust-bitcoin we have started pinning our nightly compiler (which is where we run clippy) and we have a cronjob that runs every so often (was daily, but we reduced it to twice a week, and realistically it'd be fine if it were twice a year) which opens a PR to bump the pin.

If the PR passes CI we know that we can accept it and increase our pin. If it fails CI then we can see that there's an upcoming lint failure and we can address it at our own pace.

[danielabrozzoni]

In rust-bitcoin we have started pinning our nightly compiler (which is where we run clippy) and we have a cronjob that runs every so often (was daily, but we reduced it to twice a week, and realistically it'd be fine if it were twice a year) which opens a PR to bump the pin.

@apoelstra thanks for sharing! Definitely something we should do in the future, we did forget to bump the pin on CI a few times. Opened a new good first issue: https://github.com/bitcoindevkit/bdk/issues/1433