A build cache serves a dependency rebuild after a branch switch or a `git clean` instead of
recompiling it, which is most of what the in-loop verify spends. It stays a convenience and
never a requirement: with nothing on `PATH` nothing is exported and a clone needs only stable
Rust and a C compiler, exactly as before.
`check.sh`, `gates.sh` and `smoke.sh` source it, so the loops pick it up without being asked
and a machine that would rather not have it sets `BENILLA_NO_SCCACHE=1`. An existing
`RUSTC_WRAPPER` wins. Only rustc is wrapped — the vendored Lua build wants its own
`CC="sccache cc"`, so it is not cached by this.
