Fix JRUBY-6603

To avoid attempting to read the ENV hash while it might be updated
elsewhere, I added a dup call. This is not 100% perfect if dup is
not threadsafe (unsure) but it was able to pass the given
contrived example.

src/org/jruby/util/ShellLauncher.java

@@ -215,7 +215,7 @@ private static String[] getCurrentEnv(Ruby runtime) {
     }
 
     private static String[] getCurrentEnv(Ruby runtime, Map mergeEnv) {
-        RubyHash hash = (RubyHash)runtime.getObject().getConstant("ENV");
+        RubyHash hash = (RubyHash)runtime.getObject().getConstant("ENV").dup();
         String[] ret;
 
         if (mergeEnv != null && !mergeEnv.isEmpty()) {