[build-script] Fix missing install-symroot flag

Author: shahmishal

utils/build-script

@@ -890,6 +890,7 @@ the number of parallel build jobs to use""",
         os.path.join(SWIFT_SOURCE_ROOT, "swift", "utils", "build-script-impl"),
         "--build-dir", build_dir,
         "--install-prefix", os.path.abspath(args.install_prefix),
+        "--install-symroot", os.path.abspath(args.install_symroot),
         "--host-target", args.host_target,
         "--host-cc", host_clang.cc,
         "--host-cxx", host_clang.cxx,