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

de2763a 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 de2763a
  storopoli:
    ACK de2763a
  notmandatory:
    ACK de2763a
  oleonardolima:
    ACK de2763a

Tree-SHA512: 73cad29a5ff437290aca8f85a011c4f5fc4d9ff5755f3d3ef9fa1820f5631eda857b1a67955adfc6ef98145958c674cc09f7613b96f38cc30c75a656d872edbc

.github/workflows/cont_integration.yml

@@ -118,7 +118,7 @@ jobs:
       - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1
         with:
-            toolchain: stable
+            toolchain: 1.78.0
             components: clippy
             override: true
       - name: Rust Cache