fix(cargo-wdk): disable Cargo build-dir v2 layout via environment variable

[svasista-ms]

TBD

[Copilot]

Pull request overview

This PR updates cargo-wdk’s build flow to explicitly disable Cargo’s “build-dir v2 / new target-dir layout” via an environment variable during cargo build, ensuring cargo-wdk continues to observe the expected output layout.

Changes:

• Passes CARGO_UNSTABLE_BUILD_DIR_NEW_LAYOUT=false into the cargo build invocation from BuildTask.
• Adds a dedicated unit test asserting the environment variable is forwarded to the command executor.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.86%. Comparing base (66223e9) to head (02c34ca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #708      +/-   ##
==========================================
- Coverage   80.47%   75.86%   -4.62%     
==========================================
  Files          26       31       +5     
  Lines        5722     6174     +452     
  Branches     5722     6174     +452     
==========================================
+ Hits         4605     4684      +79     
- Misses        989     1354     +365     
- Partials      128      136       +8     

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