Merge bitcoindevkit/bdk#1524: ci: pin tokio to 1.38.1 to support MSRV 1.63

28d75304e1319f49ab72c582f03a916ee40463dd ci: pin tokio to 1.38.1 to support MSRV 1.63 (Steve Myers)

Pull request description:

  ### Description

  The latest tokio minor version update from 1.38.1 to 1.39.1 changed it's MSRV from 1.63.0 to 1.70.0, breaking our CI MSRV 1.63 testing. This PR pins `tokio` back to 1.38.1 for our CI MSRV testing.

  ### Notes to the reviewers

  https://github.com/tokio-rs/tokio/pull/6645

  https://crates.io/crates/tokio/versions

  ### 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:
  LagginTimes:
    ACK 28d75304e1319f49ab72c582f03a916ee40463dd
  ValuedMammal:
    ACK 28d75304e1319f49ab72c582f03a916ee40463dd
  oleonardolima:
    ACK 28d75304e1319f49ab72c582f03a916ee40463dd

Tree-SHA512: 9eef750422cb8c3faa15771438e790afae44362d7c77e1d36cf70816ac7dabdd2e408e14502836ec86f845a2f1fe82905000c681fb6b4e873b036a41a280abd9