calibre: fix build until #472656 lands on master

This applies a workaround suggested by @wrvsrv. Unfortunately, the
correct fix (#472656) requires a mass rebuild so it'll take a while to
pass through staging.

fixes #493843

pkgs/by-name/ca/calibre/package.nix

@@ -184,6 +184,9 @@ stdenv.mkDerivation (finalAttrs: {
   installPhase = ''
     runHook preInstall
 
+    # Work around #493843 until #493988 lands on master.
+    export QMAKE="${qt6.qtbase}/bin/qmake"
+
     python setup.py install --root=$out \
       --prefix=$out \
       --libdir=$out/lib \