gnu: milkytracker: Update to 1.0.0.

* gnu/packages/music.scm (milkytracker): Update to 1.0.0.
[source]: Change uri; add file-name; add snippet to remove non-FSDG
compliant data.
[build-system]: Change from gnu-build-system to cmake-build-system.
[arguments]: Remove #:make-flags; disable tests (which are no longer
present).
[inputs]: Change sdl to sdl2.