message: Update tf-nightly to use keras-nightly 2.6.
  The release version is still using 2.5.
diff: @@ -120,7 +120,7 @@ if 'tf_nightly' in project_name:
  elif 'tensorflow_estimator' in pkg:
  REQUIRED_PACKAGES[i] = 'tf-estimator-nightly ~= 2.5.0.dev'
  elif 'keras' in pkg and 'keras_preprocessing' not in pkg:
  - REQUIRED_PACKAGES[i] = 'keras-nightly ~= 2.5.0.dev'
  + REQUIRED_PACKAGES[i] = 'keras-nightly ~= 2.6.0.dev'
  # grpcio does not build correctly on big-endian machines due to lack of
  