[build] Make xtask coverage repeatable by cleaning llvm-cov artifacts; add opt-out flags

[Copilot]

cargo xtask coverage (and precheck --coverage) could fail on reruns due to stale *.profraw data under target/llvm-cov-target. This change makes coverage runs repeatable by default and adds explicit CLI opt-outs where needed.

• Coverage command behavior

  • xtask coverage now executes cargo llvm-cov clean --workspace before running coverage jobs/reports.
  • This removes stale llvm-cov artifacts that caused subsequent report generation to fail.

• New CLI opt-out on coverage

  • Added --skip-clean to cargo xtask coverage to bypass the pre-run clean step when desired.

• Precheck integration

  • Added --skip-clean to cargo xtask precheck.
  • When precheck invokes coverage, it now forwards this flag to the coverage task.

if !self.skip_clean {
    cmd!(sh, "cargo llvm-cov clean --workspace").run()?;
}

[Copilot]

Pull request overview

This PR makes cargo xtask coverage (and cargo xtask precheck --coverage) repeatable across reruns by default by cleaning stale cargo-llvm-cov artifacts up front, while adding explicit opt-out flags for cases where preserving artifacts is desired.

Changes:

• Added a default pre-run cargo llvm-cov clean --workspace step to xtask coverage, gated by a new --skip-clean flag.
• Added --skip-coverage-clean to xtask precheck and forwarded it to xtask coverage.
• Added focused unit tests validating the new Coverage clap flag default and opt-out behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
xtask/src/precheck.rs	Adds --skip-coverage-clean and forwards it when invoking the coverage xtask.
xtask/src/coverage.rs	Adds --skip-clean, runs cargo llvm-cov clean --workspace by default, and includes clap parsing unit tests.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[zimmy87]

lgtm

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.