Make :graaljs and :nashorn main files work without java package globals

output-main-file for nashorn and graaljs targets make use of java.io.File. This makes the produced artifact not work when trying to execute it with js.java-package-globals (see CLJS-3087)

It should either just concat the path strings with a forward slash, which should also work just fine on Windows as per https://docs.oracle.com/javase/8/docs/technotes/guides/scripting/nashorn/shell.html (unfortunately I don't have a machine to verify) or use the Java.type approach used in graaljs_load