Merge bitcoindevkit/bdk#2080: ci: Pin coverage toolchain to `nightly-2025-11-27`

Merge bitcoindevkit/bdk#2080: ci: Pin coverage toolchain to `nightly-2025-11-27`

0e8849cea6a3c9965ef145f0449167f2e6b2d9d7 ci(coverage): Pin toolchain to `nightly-2025-11-27` (valued mammal)

Pull request description:

  ### Description

  `code_coverage.yml` started failing as can be seen in a [recent CI run](https://github.com/bitcoindevkit/bdk/actions/runs/19946035484/job/57195537541). As a workaround I set the toolchain in the coverage workflow to `nightly-2025-11-27` which is the version of the compiler in use the last time there was a successful coverage run.

  The cause is unknown (to me) and likely doesn't originate from a crate in `bdk`. Assuming that a fix can be found, we can eventually revert this or otherwise keep the toolchain in `code_coverage.yml` up to date as needed.

  ### Changelog notice

  ```md
  ci: Set code coverage toolchain to `nightly-2025-11-27`
  ```

  ### Checklists

  #### All Submissions:

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

ACKs for top commit:
  luisschwab:
    ACK 0e8849cea6a3c9965ef145f0449167f2e6b2d9d7
  thunderbiscuit:
    ACK 0e8849cea6a3c9965ef145f0449167f2e6b2d9d7.
  notmandatory:
    ACK 0e8849cea6a3c9965ef145f0449167f2e6b2d9d7

Tree-SHA512: 73db780c2ca5afe3b8b65b4b9e2daca7ec984e0f72db87719f9b8e1de6f300314e416f16658e3aba1dd35e1101b1f6be79978b95e33c0b57834a639953778b40