Donald Trump declared war on the United Auto Workers head Shawn Fain late Sunday night after the union leader took potshots at him earlier in the day on “Face the Nation.” While on the CBS program, Fain told host Margaret Brennan, “I can’t fathom any union would support Donald Trump for president,” before later adding that during his presidency, “he cycled through White House staff like toilet paper.” Previously Fain had called Trump a “scab.” That, combined with Fain’s endorsement of President Joe Biden last week, appears to have gotten under the four-times indicted ex-president’s skin who la…