mp3gain: Fix output path bin directory.

Signed-off-by: aszlig <[e-mail]>

pkgs/applications/audio/mp3gain/default.nix

···
   buildFlags = [ "OSTYPE=linux" ];
 
   installPhase = ''
-    mkdir -p $out/usr/bin
-    cp mp3gain $out/usr/bin
+    install -vD mp3gain "$out/bin/mp3gain"
   '';
 
   meta = {