Merge bitcoindevkit/bdk#539: [ci] Pin nightly docs workflow rust version to nightly...

07c270db03bac07604911e6bd253c18713de0ab0 [ci] Pin nightly docs workflow rust version to nightly-2022-01-25 (Steve Myers)

Pull request description:

  ### Description

  Pin nightly docs workflow rust version to `nightly-2022-01-25` to fix #538 .

  ### Notes to the reviewers

  The nightly docs should be changed to the `stable` version once it supports `feature(doc_cfg)`.

  ### 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)

  #### Bugfixes:

  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  afilini:
    ACK 07c270db03bac07604911e6bd253c18713de0ab0

Tree-SHA512: a8e5de051e1963c10d686d5cb5d1785e20f6cd322b9bfcf2d2f5c381d9124a9b8518671dfbedb11550bdfa66c8c64ad505c7e1604e2bfe23600de39cb20467dd