qutebrowser: downgrade Qt to 5.14

There are a number of unresolved issues with Qt 5.15, the most
important one being the inability to properly restore sessions[1].
For now it's better to stay at the earlier version.

[1]: qutebrowser/qutebrowser#5359

pkgs/top-level/all-packages.nix

@@ -22967,11 +22967,11 @@ in
     pkgs_ = pkgs.extend(_: prev: {
       pythonInterpreters = prev.pythonInterpreters.override(oldAttrs: {
         pkgs = oldAttrs.pkgs.extend(_: _: {
-          inherit (pkgs) qt5 libsForQt5;
+          inherit (pkgs) qt5 libsForQt514;
         });
       });
     });
-  in pkgs_.libsForQt5.callPackage ../applications/networking/browsers/qutebrowser { };
+  in pkgs_.libsForQt514.callPackage ../applications/networking/browsers/qutebrowser { };
 
   qxw = callPackage ../applications/editors/qxw {};