discord: fix desktop icon location

${out}/share/pixmaps and $XDG_DATA_DIRS/share/pixmaps are not part
of the freedesktop icon theme spec. /usr/share/pixmaps is. There
are some packages that have been patched to recognize this non
standard path. Those packages may be depending this path so here
we only add the equivalent hicolor theme fallback path.

Without this change the discord icon won't be picked up by apps
attempting to follow the lookup pattern in the freedesktop spec.

pkgs/applications/networking/instant-messengers/discord/linux.nix

@@ -71,7 +71,7 @@ stdenv.mkDerivation rec {
   ];
 
   installPhase = ''
-    mkdir -p $out/{bin,opt/${binaryName},share/pixmaps}
+    mkdir -p $out/{bin,opt/${binaryName},share/pixmaps,share/icons/hicolor/256x256/apps}
     mv * $out/opt/${binaryName}
 
     chmod +x $out/opt/${binaryName}/${binaryName}
@@ -89,7 +89,9 @@ stdenv.mkDerivation rec {
     ln -s $out/opt/${binaryName}/${binaryName} $out/bin/${
       lib.strings.toLower binaryName
     } || true
+
     ln -s $out/opt/${binaryName}/discord.png $out/share/pixmaps/${pname}.png
+    ln -s $out/opt/${binaryName}/discord.png $out/share/icons/hicolor/256x256/apps/${pname}.png
 
     ln -s "${desktopItem}/share/applications" $out/share/
   '';