nix: add mainProgram

Silences warnings like "getExe: Package
"copyparty-1.16.15" does not have the
meta.mainProgram attribute. We'll assume that the
main program has the same name for now, but this
behavior is deprecated, because it leads to
surprising errors when the assumption does not
hold. If the package has a main program, please
set `meta.mainProgram` in its definition to make
this warning go away. Otherwise, if the package
does not have a main program, or if you don't
control its definition, use getExe' to specify
the name to the program, such as lib.getExe' foo
"bar"."

contrib/package/nix/copyparty/default.nix

@@ -64,4 +64,5 @@ in stdenv.mkDerivation {
       --set PATH '${lib.makeBinPath ([ utillinux ] ++ lib.optional withMediaProcessing ffmpeg)}:$PATH' \
       --add-flags "$out/share/copyparty-sfx.py"
   '';
+  meta.mainProgram = "copyparty";
 }