[Checkins] [zopefoundation/zope.testrunner] e78860: cmd.exe doesn't like undefined environment variables?

[Marius Gedminas]

  Branch: refs/heads/py38
  Home:   https://github.com/zopefoundation/zope.testrunner
  Commit: e7886067f33de93a1ce5b599ef749c51f448c9fd
      https://github.com/zopefoundation/zope.testrunner/commit/e7886067f33de93a1ce5b599ef749c51f448c9fd
  Author: Marius Gedminas <marius at gedmin.as>
  Date:   2019-02-12 (Tue, 12 Feb 2019)

  Changed paths:
    M appveyor.yml

  Log Message:
  -----------
  cmd.exe doesn't like undefined environment variables?