Ensure proper access rights in `lein jar`

Fix for #1724

The error output will look like this:

```
$ lein jar
java.io.IOException: Permission denied. Please check your access rights for /testing/path/
 at leiningen.jar$jar.invoke (jar.clj:335)
    leiningen.jar$jar.invoke (jar.clj:346)
    ...
    clojure.main.main (main.java:37)
```