Use unique coverage file per env

tox.ini

@@ -20,6 +20,8 @@ setenv =
     gcp: CB_TEST_PROVIDER=gcp
     openstack: CB_TEST_PROVIDER=openstack
     mock: CB_TEST_PROVIDER=mock
+    # https://github.com/nedbat/coveragepy/issues/883#issuecomment-650562896
+    COVERAGE_FILE=.coverage.{envname}
 passenv =
     PYTHONUNBUFFERED
     aws: CB_IMAGE_AWS CB_INSTANCE_TYPE_AWS CB_PLACEMENT_AWS AWS_ACCESS_KEY AWS_SECRET_KEY