gnu: Use 'ghostscript-gs' in packages that need the 'gs' command.

gnu: Use 'ghostscript-gs' in packages that need the 'gs' command.

Fixes a regression introduced in
eb354bdacbf4154ec66038dac07f19bf4ced1fad.

Reported by Andreas Enge <andreas@enge.fr>.

* gnu/packages/gv.scm (gv)[propagated-inputs]: Change from GHOSTSCRIPT/X
to GHOSTSCRIPT-GS/X.
* gnu/packages/music.scm (lilypond): Use GHOSTSCRIPT-GS instead of
GHOSTSCRIPT.
* gnu/packages/ocaml.scm (unison): Likewise.
* gnu/packages/plotutils.scm (asymptote): Likewise.
* gnu/packages/scheme.scm (chez-scheme): Likewise.