Conversion doesn't work in >5000 builds

[kolnogorov]

If you try to convert something, you'll get an "Conversion mode is not specified" error, even though the "mode" parameter is specified.

For example:

zxtune123 --convert mode=psg,filename=music.psg music.pt3

Will always throw:

Conversion mode is not specified.

In 4990 build (and eariler) everything is ok.

Tested on Ubuntu 20.04, macOS 12.5.1 and Windows 11.