Consistently use bin/qmk when that script is called (#12286)

* Pass QMK_BIN down to build_keyboard.mk

* choose the correct qmk script

bin/qmk

@@ -73,6 +73,7 @@ def main():
     """
     # Change to the root of our checkout
     os.environ['ORIG_CWD'] = os.getcwd()
+    os.environ['DEPRECATED_BIN_QMK'] = '1'
     os.chdir(qmk_dir)
 
     print('Warning: The bin/qmk script is being deprecated. Please install the QMK CLI: python3 -m pip install qmk', file=sys.stderr)