text: Title: Run xonsh with `-DVAR VAL`: ValueError: not enough values to unpack
  Body: ```xsh
  xonsh --no-rc --no-env -DCOLOR_OUTPUT 0  # space between -DVAR and VALUE
  ```
  ```xsh
  Traceback (most recent call last):
    File "/Users/pc/.local/xonsh-env/lib/python3.12/site-packages/xonsh/main.py", line 478, in main
      args = premain(argv)
             ^^^^^^^^^^^^^
    File "/Users/pc/.local/xonsh-env/lib/python3.12/site-packages/xonsh/main.py", line 420, in premain
      env.update([x.split("=", 1) for x in args.defines])
    File "<frozen _collections_abc>", line 987, in update
  ValueError: not enough values to unpack (expected 2, got 1)
  Xonsh encountered an issue during launch
  ```
  
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