Use the new 'graalvm_os' substitution.

Author: ansalond

mx.truffleruby/mx_truffleruby.py

@@ -132,7 +132,7 @@ def ruby_testdownstream_sulong(args):
     name='TruffleRuby',
     short_name='rby',
     dir_name='ruby',
-    standalone_dir_name='truffleruby-<version>-<os>-<arch>',
+    standalone_dir_name='truffleruby-<version>-<graalvm_os>-<arch>',
     license_files=['LICENSE_TRUFFLERUBY.md'],
     third_party_license_files=['3rd_party_licenses_truffleruby.txt'],
     truffle_jars=[