♻️ Update tests/test_select_gen.py, pass environment variables, needed for NixOS nixpkgs (#969)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_select_gen.py

@@ -1,3 +1,4 @@
+import os
 import subprocess
 import sys
 from pathlib import Path
@@ -9,9 +10,11 @@ root_path = Path(__file__).parent.parent
 
 @needs_py39
 def test_select_gen() -> None:
+    env = os.environ.copy()
+    env["CHECK_JINJA"] = "1"
     result = subprocess.run(
         [sys.executable, "scripts/generate_select.py"],
-        env={"CHECK_JINJA": "1"},
+        env=env,
         check=True,
         cwd=root_path,
         capture_output=True,