audacity: apply Arch patch to avoid vulnerable ffmpeg

Now it builds with the latest branch. /cc #12437.

pkgs/applications/audio/audacity/default.nix

···
 { stdenv, fetchurl, wxGTK, pkgconfig, gettext, gtk, glib, zlib, perl, intltool,
-  libogg, libvorbis, libmad, alsaLib, libsndfile, soxr, flac, lame,
+  libogg, libvorbis, libmad, alsaLib, libsndfile, soxr, flac, lame, fetchpatch,
   expat, libid3tag, ffmpeg, soundtouch /*, portaudio - given up fighting their portaudio.patch */
   }:
 
···
     url = "https://github.com/audacity/audacity/archive/Audacity-${version}.tar.gz";
     sha256 = "15c5ff7ac1c0b19b08f4bdcb0f4988743da2f9ed3fab41d6f07600e67cb9ddb6";
   };
+  patches = [(fetchpatch {
+    name = "new-ffmpeg.patch";
+    url = "https://projects.archlinux.org/svntogit/packages.git/plain/trunk"
+      + "/audacity-ffmpeg.patch?h=packages/audacity&id=0c1e35798d4d70692";
+    sha256 = "19fr674mw844zmkp1476yigkcnmb6zyn78av64ccdwi3p68i00rf";
+  })];
 
   preConfigure = /* we prefer system-wide libs */ ''
     mv lib-src lib-src-rm

pkgs/top-level/all-packages.nix

···
 
   audacious = callPackage ../applications/audio/audacious { };
 
-  audacity = callPackage ../applications/audio/audacity {
-    ffmpeg = ffmpeg_2_2;
-  };
+  audacity = callPackage ../applications/audio/audacity { };
 
   audio-recorder = callPackage ../applications/audio/audio-recorder { };