Check Cargo.lock freshness

[reez]

Description

Adds a locked Cargo check to Rust CI so stale Cargo.lock changes fail clearly.

If the lockfile is out of sync, CI now suggests running cargo check.

Notes to the reviewers

First commit adds cargo check --locked to Rust CI so stale Cargo.lock changes fail.

Second commit wraps the same command in a small shell block so CI can print a clearer hint to refresh the lockfile. (Totally dont have to do it this way so I can remove this commit if we want!)

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:

Changelog

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[reez]

updated d23b2db to have a better hint @thunderbiscuit

[thunderbiscuit]

ACK d23b2db.