treewide: \xc2\xa0 ->

I have no idea what this escape sequence even is, but it breaks the nix parser with cryptic errors if not used in a comment.
A friend let me know MacOS is prone to input weird spaces, not sure if that is the source.

Candidates were located and created with:

    chr="$(echo -e '\xc2\xa0')"; rg -F "$chr" -l | xe sd -F "$chr" " "

There are some examples left, most being example output from `tree` in various markdown documents, some patches which we can't really touch, and `pkgs/tools/nix/nixos-render-docs/src/tests/test_commonmark.py` which I'm not sure if should be addressed

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -8,7 +8,7 @@ let
    * to a menuentry for use in grub.
    *
    *  * defaults: {name, image, params, initrd}
-   *  * options: [ option... ]
+   *  * options: [ option... ]
    *  * option: {name, params, class}
    */
   menuBuilderGrub2 =