[pytest] Use knobs to setup fresh cache for testing (#6705)

With the new knobs module we have a more pythonic way to setup the
cache. Other than being 'pythonic', switching to use knobs ensures the
fresh cache will override any previously set cache path (e.g. if you
have generic initialization logic which sets up these knobs, which we do
have at Meta).

Author: danzimm

python/test/unit/conftest.py

@@ -1,4 +1,3 @@
-import os
 import pytest
 import tempfile
 from typing import Optional, Set
@@ -20,11 +19,10 @@ def device(request):
 @pytest.fixture
 def fresh_triton_cache():
     with tempfile.TemporaryDirectory() as tmpdir:
-        try:
-            os.environ["TRITON_CACHE_DIR"] = tmpdir
+        from triton import knobs
+        with knobs.cache.scope():
+            knobs.cache.dir = tmpdir
             yield tmpdir
-        finally:
-            os.environ.pop("TRITON_CACHE_DIR", None)
 
 
 def _fresh_knobs_impl(monkeypatch, skipped_attr: Optional[Set[str]] = None):