noisetorch: add `meta.mainProgram`

the `lib.getExe`'s feature to use the package's name as the binary is
deprecated, and `pkgs/README.md` says:
> `meta.mainProgram` must be set when appropriate.

This just adds the `meta.mainProgram` to `noisetorch` as it's the only
binary.

pkgs/applications/audio/noisetorch/default.nix

···
     license = licenses.gpl3Plus;
     platforms = platforms.linux;
     maintainers = with maintainers; [ panaeon lom ];
+    mainProgram = "noisetorch";
   };
 }