Up-to-date checks do not work correctly for checkFormat<sourceSet> tasks

[wilkinsona]

The checkFormat<sourceSet> tasks are always executed, even if the code is unchanged since the previous execution and no formatting problems exist. The following info logging is produced:

> Task :checkFormatMain
Task ':checkFormatMain' is not up-to-date because:
  Task has not declared any outputs despite executing actions.
:checkFormatMain (Thread[Task worker for ':',5,main]) completed. Took 0.018 secs.
:checkFormatTest (Thread[Task worker for ':',5,main]) started.