beets: patch ffmpeg invocation in convert plugin

The default convert configuration invokes ffmpeg, so this patches in the
right storepath. Since it patches the shlex split, even user config will
use the correct path. kudos @aszlig.

pkgs/tools/audio/beets/default.nix

@@ -4,6 +4,7 @@
 , enableAcousticbrainz ? true
 , enableAcoustid       ? true
 , enableBadfiles       ? true, flac ? null, mp3val ? null
+, enableConvert        ? true, ffmpeg ? null
 , enableDiscogs        ? true
 , enableEchonest       ? true
 , enableEmbyupdate     ? true
@@ -22,6 +23,7 @@
 
 assert enableAcoustid    -> pythonPackages.pyacoustid     != null;
 assert enableBadfiles    -> flac != null && mp3val != null;
+assert enableConvert     -> ffmpeg != null;
 assert enableDiscogs     -> pythonPackages.discogs_client != null;
 assert enableEchonest    -> pythonPackages.pyechonest     != null;
 assert enableFetchart    -> pythonPackages.responses      != null;
@@ -38,6 +40,7 @@ let
     acousticbrainz = enableAcousticbrainz;
     badfiles = enableBadfiles;
     chroma = enableAcoustid;
+    convert = enableConvert;
     discogs = enableDiscogs;
     echonest = enableEchonest;
     embyupdate = enableEmbyupdate;
@@ -52,7 +55,7 @@ let
   };
 
   pluginsWithoutDeps = [
-    "bench" "bpd" "bpm" "bucket" "convert" "cue" "duplicates" "edit" "embedart"
+    "bench" "bpd" "bpm" "bucket" "cue" "duplicates" "edit" "embedart"
     "filefilter" "freedesktop" "fromfilename" "ftintitle" "fuzzy" "ihate"
     "importadded" "importfeeds" "info" "inline" "ipfs" "keyfinder" "lyrics"
     "mbcollection" "mbsubmit" "mbsync" "metasync" "missing" "permissions" "play"
@@ -96,6 +99,7 @@ in buildPythonApplication rec {
               || enableEmbyupdate
               || enableAcousticbrainz)
                                    pythonPackages.requests2
+    ++ optional enableConvert      ffmpeg
     ++ optional enableDiscogs      pythonPackages.discogs_client
     ++ optional enableEchonest     pythonPackages.pyechonest
     ++ optional enableLastfm       pythonPackages.pylast
@@ -131,6 +135,8 @@ in buildPythonApplication rec {
       s,"flac","${flac.bin}/bin/flac",
       s,"mp3val","${mp3val}/bin/mp3val",
     }' beetsplug/badfiles.py
+  '' + optionalString enableConvert ''
+    sed -i -e 's,\(util\.command_output(\)\([^)]\+\)),\1[b"${ffmpeg.bin}/bin/ffmpeg" if args[0] == b"ffmpeg" else args[0]] + \2[1:]),' beetsplug/convert.py 
   '' + optionalString enableReplaygain ''
     sed -i -re '
       s!^( *cmd *= *b?['\'''"])(bs1770gain['\'''"])!\1${bs1770gain}/bin/\2!