This adds s2tc as an input to mesa, and uses a mesa-without-s2tc package
as an input to s2tc to solve a cyclic dependency.

The resulting s2tc package does *not* link to any libraries in the
mesa-without-s2tc package (it's not a requisite as per "guix gc -R"), so
this should be safe, as in the user won't end up with two different
libGL images in memory.