fix(ci): pin yoke-derive to 0.8.2 for MSRV

[tvpeter]

Description

yoke-derive 0.8.3 uses inherent str::from_utf8, which needs rustc 1.87+. That breaks MSRV 1.85 CI:

error[E0599]: no function or associated item named `from_utf8` found for type `str` in the current scope
Error:    --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/yoke-derive-0.8.3/src/lib.rs:202:32
    |
    |         let underscores = str::from_utf8(&underscores).expect("_ is ASCII and thus UTF-8");
    |                                ^^^^^^^^^ function or associated item not found in `str`

This patch pins yoke-derive to 0.8.2 in ci/pin-msrv.sh

To test locally, use the below recipe:

./ci/pin-msrv.sh
RUSTFLAGS="--cfg bdk_wallet_unstable" \
RUSTDOCFLAGS="--cfg bdk_wallet_unstable" \
cargo +1.85.0 test --workspace --all-features

Notes to the reviewers

This is an interim fix pending #494.

Before submitting

• I followed the contribution guidelines
• This PR breaks the existing API

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.66%. Comparing base (6fc6846) to head (16c8423).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
- Coverage   81.91%   81.66%   -0.25%     
==========================================
  Files          25       25              
  Lines        6535     6339     -196     
  Branches      302      302              
==========================================
- Hits         5353     5177     -176     
+ Misses       1075     1055      -20     
  Partials      107      107              

Flag	Coverage Δ
rust	81.66% <ø> (-0.25%)	⬇️

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[noahjoeris]

tACK 16c8423