https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/467070
---------------------------------------------
Traceback (most recent call last):
  File "/b/rr/tmpp0nlQx/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/step_runner.py", line 181, in open_step
    step_env = _merge_envs(os.environ, (rendered_step.config.env or {}))
  File "/b/rr/tmpp0nlQx/rw/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/step_runner.py", line 670, in _merge_envs
    result[str(k)] = str(v) % original
TypeError: float argument required, not instance
---------------------------------------------

This was caused by https://chromium-review.googlesource.com/c/517847 which did a;

    ninja_env['NINJA_STATUS'] = (
        '%e [%s/%t %p - %r running, current %c p/s, overall %o p/s] ')

This passed the training / testing - but failed on production.

This is caused by _merge_env in step_runner.py;
---------------------------------------------
def _merge_envs(original, override):
  """Merges two environments.

  Returns a new environment dict with entries from |override| overwriting
  corresponding entries in |original|. Keys whose value is None will completely
  remove the environment variable. Values can contain %(KEY)s strings, which
  will be substituted with the values from the original (useful for amending, as
  opposed to overwriting, variables like PATH).
  """
  result = original.copy()
  if not override:
    return result
  for k, v in override.items():
    if v is None:
      if k in result:
        del result[k]
    else:
      result[str(k)] = str(v) % original
  return result
---------------------------------------------