Merge bitcoindevkit/bdk#1430: ci: Pin clippy to rust 1.78.0

Merge bitcoindevkit/bdk#1430: ci: Pin clippy to rust 1.78.0

de2763a4b88e65b1c67a06148b4f9e012e561c5a ci: Pin clippy to rust 1.78.0 (valued mammal)

Pull request description:

  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](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [ ] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  danielabrozzoni:
    ACK de2763a4b88e65b1c67a06148b4f9e012e561c5a
  storopoli:
    ACK de2763a4b88e65b1c67a06148b4f9e012e561c5a
  notmandatory:
    ACK de2763a4b88e65b1c67a06148b4f9e012e561c5a
  oleonardolima:
    ACK de2763a4b88e65b1c67a06148b4f9e012e561c5a

Tree-SHA512: 73cad29a5ff437290aca8f85a011c4f5fc4d9ff5755f3d3ef9fa1820f5631eda857b1a67955adfc6ef98145958c674cc09f7613b96f38cc30c75a656d872edbc