Fix for passing snippets as zephyr extra modules, in nix build scripts

[ScatteredRay]

Hi,

I was trying to add zmk-usb-logging to my build config that uses the workflows from the glove80-zmk-config repo. And I was running into problems, digging in, I wasn't able to figure out how to pass in a snippet using the snippets field which get's passed in as -DZMK_EXTRA_MODULES. Changing it to -DSNIPPET fixes my issue, and I'm now able to make a def like so:

  glove80_left = zmk.override {
    board = "glove80_lh";
    keymap = ./config/glove80.keymap;
    snippets = [
      "zmk-usb-logging"
    ];
  };

So, perhaps this is just an error that should be fixed?