qt6.qtmultimedia: Fix failure to load libva.so

QtMultimedia 6.6.0 would select dynamic VAAPI on linux, then warns
during build (even though it chose this on purpose):

> QT_FEATURE_vaapi is found but ffmpeg doesn't include vaapi,
> however dynamic symbols resolve is possible

The nuisance warning was fixed for 6.7 and backported to 6.6.1:
https://codereview.qt-project.org/c/qt/qtmultimedia/+/517333

However, tracing it helped me figure out why vaapi actually wasn't
working: nix doesn't end up with an rpath such that dlopen("va")
can actually find libva.so in the nix store, thus failing at runtime:

> qt.multimedia.plugin: loading backend "ffmpeg"
> qt.core.library: "/nix/store/i9fkjks6dfjj1p9qvj5633sxbrf5rbd8-qtmultimedia-6.6.1/lib/qt-6/plugins/multimedia/libffmpegmediaplugin.so" loaded library
> qt.multimedia.ffmpeg.libsymbolsresolver: Start VAAPI symbols resolving: 39 symbols
> qt.core.library: "va" cannot load: Cannot load library va: (va: cannot open shared object file: No such file or directory)
> qt.multimedia.ffmpeg.libsymbolsresolver: Couldn't load VAAPI library

pkgs/development/libraries/qt-6/modules/qtmultimedia.nix

@@ -33,6 +33,8 @@ qtModule {
     ++ lib.optionals stdenv.isLinux [ gstreamer gst-plugins-base gst-plugins-good gst-libav gst-vaapi ]
     ++ lib.optionals stdenv.isDarwin [ VideoToolbox ];
 
+  cmakeFlags = [ "-DENABLE_DYNAMIC_RESOLVE_VAAPI_SYMBOLS=0" ];
+
   env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin
     "-include AudioToolbox/AudioToolbox.h";
   NIX_LDFLAGS = lib.optionalString stdenv.isDarwin