Set PIP_USE_PEP517 in installer command so as not to influence the tests with that setting.

tools/tox_pip.py

@@ -68,6 +68,8 @@ def disable_python_requires():
 
 
 def run(args):
+    os.environ['PIP_USE_PEP517'] = 'true'
+
     if is_install_self(args):
         remove_setuptools()
         bootstrap()

tox.ini

@@ -18,7 +18,6 @@ install_command = {[helpers]pip} install {opts} {packages}
 list_dependencies_command = {[helpers]pip} freeze --all
 setenv =
 	COVERAGE_FILE={toxworkdir}/.coverage.{envname}
-	PIP_USE_PEP517=1
 # TODO: The passed environment variables came from copying other tox.ini files
 # These should probably be individually annotated to explain what needs them.
 passenv=APPDATA HOMEDRIVE HOMEPATH windir APPVEYOR APPVEYOR_* CI CODECOV_* TRAVIS TRAVIS_* NETWORK_REQUIRED