Fix a regression in 0.26.0 that caused kitty to no longer set the LANG environment variable on macOS

Happened because reading the locale uses cocoa APIs and they are not fork
safe, so it was moved to after prewarm forking, but at that point the
default child env had already been set.

Fixes #5439

kitty/child.py

@@ -168,6 +168,10 @@ def set_default_env(val: Optional[Dict[str, str]] = None) -> None:
     setattr(default_env, 'lc_ctype_set_by_user', has_lctype)
 
 
+def set_LANG_in_default_env(val: str) -> None:
+    default_env()['LANG'] = val
+
+
 def openpty() -> Tuple[int, int]:
     master, slave = os.openpty()  # Note that master and slave are in blocking mode
     os.set_inheritable(slave, True)