fixup! feat(Window.split_window): set up environment

Author: zappolowski

tests/test_window.py

@@ -1,5 +1,6 @@
 """Test for libtmux Window object."""
 import logging
+import shutil
 import time
 import typing as t
 
@@ -314,9 +315,12 @@ def test_empty_window_name(session: Session) -> None:
 
 
 def test_split_window_with_environment(session: Session) -> None:
+    env = shutil.which("env")
+    assert env is not None, "Cannot find usable `env` in Path."
+
     window = session.new_window(window_name="split_window_with_environment")
     pane = window.split_window(
-        shell="/usr/bin/env PS1='$ ' bash --norc --noprofile",
+        shell=f"{env} PS1='$ ' bash --norc --noprofile",
         environment={"ENV_VAR": "pane"},
     )
     assert pane is not None