tox: Add passenv DISABLE_CHERRY_PICK_CHECK to pep8

I4f551dc4b57905cab8aa005c5680223ad1b57639 introduced the environment
variable to disable the check-cherry-pick.sh script but forgot to allow
it to be passed into the pep8 tox env.

Change-Id: Ie8a672fd21184c810bfe9c0e3a49582189bf2111

tox.ini

@@ -51,6 +51,8 @@ commands =
 description =
   Run style checks.
 envdir = {toxworkdir}/shared
+passenv =
+  DISABLE_CHERRY_PICK_CHECK
 commands =
   {[testenv:mypy]commands}
   bash tools/flake8wrap.sh {posargs}