zotero: Add meta.mainProgram

The lib.getExe function's feature of assuming the program name from the
package is depracated, and it says to specify it here.

I was not sure whether I should do zotero itself, or .zotero-wrapped. I
put just zotero here, but I will change it if it should be the wrapped
version.

I didn't see any meta.mainProgam use the wrapped one, so I didn't use it
here.

pkgs/applications/office/zotero/default.nix

@@ -153,5 +153,6 @@ stdenv.mkDerivation rec {
     license = licenses.agpl3Only;
     platforms = [ "x86_64-linux" ];
     maintainers = with maintainers; [ i077 ];
+    mainProgram = "zotero";
   };
 }