Use session-wide common dir for schemas fixture

Avoid creating separate copies of the schemas per xdist worker, and
resulting failures on GHA Windows runners.

sdmx/testing/__init__.py

@@ -229,15 +229,18 @@ def msg(self, path):
 
 
 @pytest.fixture(scope="session")
-def installed_schemas(mock_gh_api, tmp_path_factory):
+def installed_schemas(worker_id, mock_gh_api, tmp_path_factory):
     """Fixture that ensures schemas are installed locally in a temporary directory."""
     from sdmx.format.xml.common import install_schemas
 
-    dir = tmp_path_factory.mktemp("schemas")
+    if worker_id == "master":
+        dir = tmp_path_factory.mktemp("schemas")
+    else:
+        dir = tmp_path_factory.getbasetemp().parent.joinpath("schemas")
 
     with mock_gh_api:
-        install_schemas(dir.joinpath("2.1"), Version["2.1"])
-        install_schemas(dir.joinpath("3.0"), Version["3.0.0"])
+        install_schemas(dir, Version["2.1"])
+        install_schemas(dir, Version["3.0.0"])
 
     yield dir