tox: disable pkgcore's pytest plugin for benchmark target

Since it overlaps with pkgcraft's which is what we want to use.

Author: radhermit

tox.ini

@@ -47,7 +47,11 @@ commands =
 [testenv:bench]
 skip_sdist = false
 skip_install = false
-setenv = PY_COLORS = 1
+setenv =
+    # forcibly enable pytest colors
+    PY_COLORS = 1
+    # disable pkgcore's pytest plugin
+    PYTEST_ADDOPTS = -p no:pkgcore
 deps =
     pytest-benchmark
     pkgcore