windows-sandbox: fix capture cancellation test roots

[bolinfest]

Why

The Windows Bazel job on main started failing after #24108 because one Windows-only capture test still passed cwd.as_path() to run_windows_sandbox_capture. That helper now expects the explicit workspace_roots slice introduced by #24108, so the Windows test target no longer compiled.

What Changed

• Updates legacy_capture_cancellation_is_not_reported_as_timeout to pass workspace_roots_for(cwd.as_path()).as_slice(), matching the adjacent capture test and the new runner signature.

Verification

• GitHub Actions CI is the important validation for this Windows-only compile path.
• Created quickly to get Windows CI running while the separate Ubuntu compact_resume_fork timeout is still under investigation.