Ensure full environment is passed to headless test.

Otherwise, e.g., `PYTHONPATH` is not available.

Author: QuLogic

lib/matplotlib/tests/test_backends_interactive.py

@@ -301,7 +301,7 @@ def test_lazy_linux_headless():
 
 """
     proc = subprocess.run([sys.executable, "-c", test_script],
-                          env={"MPLBACKEND": ""})
+                          env={**os.environ, "MPLBACKEND": ""})
     if proc.returncode:
         pytest.fail("The subprocess returned with non-zero exit status "
                     f"{proc.returncode}.")