gnu: make-torbrowser: Build with ffmpeg@6.

ffmpeg@8 is not supported and breaks runtime support for some codecs.  See:
<https://bugzilla.mozilla.org/show_bug.cgi?id=1962139>.

* gnu/packages/tor-browsers.scm (make-torbrowser): Build with ffmpeg-6.

Change-Id: I623beb0c181400c904b77d12cb730f0d148303ba

gnu/packages/tor-browsers.scm

@@ -240,7 +240,8 @@ Browser.")
            libxcomposite
            libxt
            libffi
-           ffmpeg
+           ;; See <https://bugzilla.mozilla.org/show_bug.cgi?id=1962139>
+           ffmpeg-6
            libvpx
            icu4c
            pixman