tox: Fix typo
tox: Fix typo

We shouldn't be using environment variables in this manner.

Change-Id: Iefaa71a195c6046c770f2f598199073a951df8a5
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/oslo.cache,openstack/oslo.cache