build: scripts/sccache.sh wraps rustc when sccache is installed, and the gates source it

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.

scripts/gates.sh

@@ -18,6 +18,9 @@ here="$(cd "$(dirname "$0")/.." && pwd)"
 root="$(git rev-parse --show-toplevel 2>/dev/null || true)"
 [ -n "$root" ] && [ -f "$root/scripts/gates.sh" ] || root="$here"
 cd "$root" || exit 1
+
+# sccache, when installed, caches the builds below; opt out with BENILLA_NO_SCCACHE=1.
+. "$root/scripts/sccache.sh"
 echo "gating: $root"
 
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