SDl2_mixer: fix MIDI playback by adding timidity paths
SDl2_mixer: fix MIDI playback by adding timidity paths

Noticed on fheroes2 package: before the change fheroes2 was not able to
play MIDI samples and complained as:
    PlayMusic:  music parameter was NULL

It happens because default search paths for 'timidity.cfg'
did not include any of nixos's paths in 'timidity/options.h':

    getenv("TIMIDITY_CFG")
    "."
    #define DEFAULT_PATH    "/etc/timidity"
    #define DEFAULT_PATH1   "/usr/share/timidity"
    #define DEFAULT_PATH2   "/usr/local/share/timidity"
    #define DEFAULT_PATH3   "/usr/local/lib/timidity"

As a result even with globally installed timidity MIDI can't be played.
abuse package also has the same problem and works it around by changing
directory to local copy of 'timidity.cfg':

    # The timidity bundled into SDL_mixer looks in . and in several global places
    # like /etc for its configuration file.
    cd @out@/etc
    exec @out@/bin/.abuse-bin "$@"

The change fixes SDL2_mixer by changing "/usr/share/timidity" path
to nix's timidity store path. Tested on fheroes2 with MIDI files.

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