fix(ci): pin tokio to 1.38.1 on MSRV step

[oleonardolima]

Description

As previously noticed on bitcoindevkit/bdk#1524, tokio raised their MSRV to 1.70.0 in their latest release, in short it was because they moved to mio 1.0 which requires 1.70.0 MSRV.

Although we don't have tokio as a direct dependency on rust-esplora-client it comes as a transitive dependency from reqwest.

This PR adds the necessary pinning on CI and updates the instruction on README.md in order to properly build on MSRV.

Notes to the reviewers

Changelog notice

• Adds another MSRV pinning step on CI for the tokio 1.38.1.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[notmandatory]

ACK 16cdd38

Thanks for fixing this here!