fetchurl: add codemirror.dlang.org to the list of dub mirrors

code.dlang.org actually went down recently, hopefully this should
make that a bit less likely to happen in the future.

pkgs/build-support/fetchurl/mirrors.nix

···
   # D DUB
   dub = [
     "https://code.dlang.org/packages/"
+    "https://codemirror.dlang.org/packages/"
   ];
 
   # Haskell Hackage