Etc is not thread-safe

* Specifically Etc.getgrgid fails the spec in spec/ruby/library/etc/getgrgid_spec.rb

Author: eregon

src/main/c/etc/etc.c

@@ -1116,6 +1116,10 @@ Init_etc(void)
 
 #ifdef HAVE_RB_EXT_RACTOR_SAFE
     RB_EXT_RACTOR_SAFE(true);
+#endif
+#ifdef TRUFFLERUBY
+    // Actually not Ractor-safe: https://bugs.ruby-lang.org/issues/21115
+    RB_EXT_RACTOR_SAFE(false);
 #endif
     mEtc = rb_define_module("Etc");
     rb_define_const(mEtc, "VERSION", rb_str_new_cstr(RUBY_ETC_VERSION));