Merge bitcoindevkit/bdk#501: Only run clippy for the stable rust version

57a1185aef82718b8e6ad1272237f4d8f644fdc2 Only run clippy for the stable rust version (Steve Myers)

Pull request description:

  ### Description

  It was decided during the team call today (2021-12-14)  to only run clippy for the stable rust version.

  ### Notes to the reviewers

  This is required to fix the below build issues when running clippy on rust version 1.46.0.

  ```shell
  cargo clippy --all-targets --features async-interface --no-default-features -- -D warnings
  ```

  ```text
  ...

  Checking bitcoincore-rpc v0.14.0
  error: unknown clippy lint: clippy::no_effect_underscore_binding
    --> src/blockchain/mod.rs:88:1
     |
  88 | #[maybe_async]
     | ^^^^^^^^^^^^^^
     |
     = note: `-D clippy::unknown-clippy-lints` implied by `-D warnings`
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
     = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

  error: unknown clippy lint: clippy::no_effect_underscore_binding
     --> src/blockchain/mod.rs:220:1
      |
  220 | #[maybe_async]
      | ^^^^^^^^^^^^^^
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
      = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
  ```

  ### 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)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

Top commit has no ACKs.

Tree-SHA512: 3fe0d2829415c7276d5339e217cefba1255c14d6d73ec0a5eff2b8072d189ffef56088623ef75f84e400d3d05e546f759b8048082b467a3738885796b3338323