vimix-cursors: fix build

The `cairosvg` executable is no longer supplied by
`python3Packages.cairosvg`, use the top-level package instead.

pkgs/by-name/vi/vimix-cursors/package.nix

···
   lib,
   fetchFromGitHub,
   stdenvNoCC,
+  cairosvg,
   inkscape,
-  python3Packages,
   xcursorgen,
 }:
 stdenvNoCC.mkDerivation {
···
   };
 
   nativeBuildInputs = [
+    cairosvg
     inkscape
-    python3Packages.cairosvg
     xcursorgen
   ];