Revert "fix(build): set CARGO_TARGET_DIR per package"

[tiran]

This reverts commit 00344bb and PR #1301.

The CARGO_TARGET_DIR setting is not needed. The Rust compiler uses a local directory in the current workspace (project) by default. The downstream issue was caused by a configuration setting in downstream.

See https://doc.rust-lang.org/cargo/reference/build-cache.html

[coderabbitai]

📝 Walkthrough

Walkthrough

The change removes the public has_rust_build_backend helper and its Rust backend detection logic. It also removes the unused Rust vendor import and automatic CARGO_TARGET_DIR handling from wheel builds. The previous Rust backend detection tests are deleted. Wheel tests now cover default Cargo target assignment, explicit target preservation, and non-Rust package handling.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly states that the pull request reverts the change that set CARGO_TARGET_DIR per package.
Description check	✅ Passed	The description explains the CARGO_TARGET_DIR reversion and its cause, which directly matches the changeset.

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