Merge bitcoindevkit/bdk#2151: fix(ci): pin toolchain, and switch to `actions-rust...

Merge bitcoindevkit/bdk#2151: fix(ci): pin toolchain, and switch to `actions-rust-lang/setup-rust-toolchain`

c99a547a27994dbc899b9aeb3c8407df7c182f46 fix(ci): pin toolchain, and switch to `actions-rust-lang/setup-rust-toolchain` (valued mammal)

Pull request description:

  ### Description

  Currently the code coverage step is failing on `release/chain-0.23.x`, this PR applies the fix that's currently in master.

  ### Changelog notice

  ```
  ### Changed

  - ci(coverage): switch to `action-rust-lang/setup-rust-toolchain`.
  - ci(coverage): pin toolchain to `nightly-2025-11-27`.

  ```

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  #### Bugfixes:

  * [x] This pull request breaks the existing API
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  luisschwab:
    ACK c99a547a27994dbc899b9aeb3c8407df7c182f46

Tree-SHA512: bf7d28fc243d6248736a5db8416bc128aeb0bf9be14360105f013e732d0a6a6113dc46a017d09655e56a46c0d485552392c554535fe3efddddf83b4b0593a631