If TOX_WORK_DIR is indicated in the environment, honor it. Closes tox-dev#20.

Author: jaraco

src/tox/config/__init__.py

@@ -1109,7 +1109,8 @@ def line_of_default_to_zero(section, name=None):
         reader.addsubstitutions(toxinidir=config.toxinidir, homedir=config.homedir)
 
         if config.option.workdir is None:
-            config.toxworkdir = reader.getpath("toxworkdir", "{toxinidir}/.tox")
+            default = os.environ.get('TOX_WORK_DIR', "{toxinidir}/.tox")
+            config.toxworkdir = reader.getpath("toxworkdir", default)
         else:
             config.toxworkdir = config.toxinidir.join(config.option.workdir, abs=True)