gnu: ardour: Add a phase to install Freedesktop files.

* gnu/packages/audio.scm (ardour): [arguments]:
 - Correct comment about --freedesktop flag.
 - Adjust pretty printing for consistency.
 - Add 'install-free-desktop-files phase to actually install the files that
   were build. Fixes bug#33585 reported by baconicsynergy.

Signed-off-by: Efraim Flashner <efraim@flashner.co.il>