Merge bitcoindevkit/bdk#811: Downgrade ubuntu to 20.04 for test_hardware_wallet CI job

Merge bitcoindevkit/bdk#811: Downgrade ubuntu to 20.04 for test_hardware_wallet CI job

11dcc14374e13c526972cd57a9ad726d183fc015 Downgrade ubuntu to 20.04 for test_hardware_wallet CI job (Steve Myers)

Pull request description:

  ### Description

  As suggested by bitcoindevkit/rust-hwi#61, downgrade ubuntu version to 20.04 (instead of using latest), to fix `test_hardware_wallet` CI job.

  ### 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

ACKs for top commit:
  danielabrozzoni:
    ACK 11dcc14374e13c526972cd57a9ad726d183fc015

Tree-SHA512: 26de7bb0e562065a9d1071fb292f8532894f921c3557c418e95b05dfff529d8e5bf1751d8b5136618d3b768f10d8af714882f7c453c91a08a9c5e9aafef976b5