test/sysroot-crates-are-unstable: Fix test when rpath is disabled

Without this environment var, the test can't run rustc to find
the sysroot path.

Author: smaeul

tests/run-make/sysroot-crates-are-unstable/Makefile

@@ -1,2 +1,4 @@
+-include ../tools.mk
+
 all:
-	'$(PYTHON)' test.py
+	env '$(HOST_RPATH_ENV)' '$(PYTHON)' test.py