qutebrowser: minor fixup regarding use of qt 514

In 3fafb02125
the Qt version used by Qutebrowser was downgraded from 5.15 to 5.14.
Let's be consistent by also setting qt514 and including a comment so one
can trace why it is used.

(Note that downgrade gave it the same Qt version as used by the Python
packages set at the time, so in principle the override could be
removed.)

pkgs/top-level/all-packages.nix

@@ -23002,7 +23002,9 @@ in
     pkgs_ = pkgs.extend(_: prev: {
       pythonInterpreters = prev.pythonInterpreters.override(oldAttrs: {
         pkgs = oldAttrs.pkgs.extend(_: _: {
-          inherit (pkgs) qt5 libsForQt514;
+          # Use 5.14 https://github.com/NixOS/nixpkgs/commit/3fafb021256bc594cecd949b3edc5bc480fc721f
+          qt5 = pkgs.qt514;
+          libsForQt5 = pkgs.libsForQt514;
         });
       });
     });