just mutants* points TMPDIR at ~/.cache for cargo-mutants' tree copies;
the tests inherited it and synced every journal append to disk, several
times slower per mutant. Cargo's [env] reaches the tests it runs, not
cargo-mutants, so the copies stay in the cache.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX
