fixup #7747: actually use the fixture for neon_env_builder (#7767)

The `= None` makes it not use the fixture.

This slipped due to last-minute changes.

test_runner/fixtures/neon_fixtures.py

@@ -1392,8 +1392,8 @@ def neon_env_builder(
     test_overlay_dir: Path,
     top_output_dir: Path,
     pageserver_virtual_file_io_engine: str,
+    pageserver_default_tenant_config_compaction_algorithm: Optional[Dict[str, Any]],
     pageserver_aux_file_policy: Optional[AuxFileStore] = None,
-    pageserver_default_tenant_config_compaction_algorithm: Optional[Dict[str, Any]] = None,
 ) -> Iterator[NeonEnvBuilder]:
     """
     Fixture to create a Neon environment for test.