[GR-15990] Disable ResolveTargetModuleForClassVariable cache for multi-context

PullRequest: truffleruby/2638

Author: bjfish

src/main/java/org/truffleruby/language/objects/classvariables/ResolveTargetModuleForClassVariablesNode.java

@@ -25,7 +25,7 @@ public static ResolveTargetModuleForClassVariablesNode create() {
 
     public abstract RubyModule execute(LexicalScope lexicalScope);
 
-    @Specialization(guards = "lexicalScope == cachedLexicalScope")
+    @Specialization(guards = { "isSingleContext()", "lexicalScope == cachedLexicalScope" })
     protected RubyModule cached(LexicalScope lexicalScope,
             @Cached("lexicalScope") LexicalScope cachedLexicalScope,
             @Cached("uncached(lexicalScope)") RubyModule cachedModule) {