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/smoke.sh

@@ -16,6 +16,9 @@ if [ -z "$root" ] || [ ! -f "$root/scripts/smoke.sh" ]; then
     root="$(cd "$(dirname "$0")/.." && pwd)"
 fi
 cd "$root" || exit 1
+
+# sccache, when installed, caches the client build below; opt out with BENILLA_NO_SCCACHE=1.
+. "$root/scripts/sccache.sh"
 echo "smoke: $root"
 
 # Resolve the account first, then unset every inherited WOW_* the case below does not keep, so