code: protected boolean isUpToDate(FilePath expectedLocation, Installable i) throws IOException, InterruptedException {
          FilePath marker = expectedLocation.child(".installedFrom");
          return marker.exists() && marker.readToString().equals(i.url);
      }
docs: Checks if the specified expected location already contains the installed version of the tool.
  
  This check needs to run fairly efficiently. The current implementation uses the source URL of {@link Installable},
  based on the assumption that released bits do not change its content.