coretest in miri: fix using unstable libtest features

Author: RalfJung

src/bootstrap/mk/Makefile.in

@@ -59,7 +59,8 @@ check-aux:
 		build-std \
 		$(BOOTSTRAP_ARGS)
 	# Run standard library tests in Miri.
-	$(Q)MIRIFLAGS="-Zmiri-strict-provenance" \
+	# We have to forward RUSTC_BOOTSTRAP so that libtest accepts the unstable args bootstrap sets.
+	$(Q)MIRIFLAGS="-Zmiri-strict-provenance -Zmiri-env-forward=RUSTC_BOOTSTRAP" \
 		$(BOOTSTRAP) miri --stage 2 \
 		library/core \
 		library/alloc \