Make check{2,3} faster by enabling parallel rustc for dcou (#34739)

Try to make check3 faster

scripts/check-dev-context-only-utils.sh

@@ -147,6 +147,7 @@ fi
 # 2. Check implicit usage of `dev-context-only-utils`-gated code in dev (=
 # test/benches) code by building in isolation from other crates, which might
 # happen to enable `dev-context-only-utils`
+export RUSTFLAGS="-Z threads=8 $RUSTFLAGS"
 if [[ $mode = "check-bins" || $mode = "full" ]]; then
   _ cargo "+${rust_nightly}" hack check --bins
 fi