Making shotcut use qt5 (5.6), not qt55.

pkgs/applications/video/shotcut/default.nix

···
 { stdenv, fetchurl, SDL, frei0r, gettext, mlt, jack1, pkgconfig, qtbase,
-qtmultimedia, qtwebkit, qtx11extras, qtwebsockets, qtquick1, qtquickcontrols,
+qtmultimedia, qtwebkit, qtx11extras, qtwebsockets, qtquickcontrols,
 qtgraphicaleffects,
 qmakeHook, makeQtWrapper }:
 
···
   };
 
   buildInputs = [ SDL frei0r gettext mlt pkgconfig qtbase qtmultimedia qtwebkit
-    qtx11extras qtwebsockets qtquick1 qtquickcontrols qtgraphicaleffects qmakeHook makeQtWrapper ];
+    qtx11extras qtwebsockets qtquickcontrols qtgraphicaleffects qmakeHook makeQtWrapper ];
 
   enableParallelBuilding = true;
 

pkgs/top-level/all-packages.nix

···
      stdenv = stdenv_32bit;
   };
 
-  shotcut = qt55.callPackage ../applications/video/shotcut { };
+  shotcut = qt5.callPackage ../applications/video/shotcut { };
 
   smplayer = qt5.callPackage ../applications/video/smplayer { };