Too many questions about texlive-texmf

[Leo Famulari]

We don't serve substitutes of texlive-texmf. We achieve this by
configuring nginx to return "410 Gone" [0].

This requires users to build it with '--fallback', and the Guix error
message even recommends this, but many users ask about it on IRC.

I think we should mark it '#:substitutable? #f' so that, IIUC, Guix will
download and build texlive-texmf from the upstream source automatically,
without stopping or requiring users to use '--fallback'.

If everyone agrees to this, exactly which package or packages should be
have this option set? There are several related variables in (gnu
packages tex) and I'm not sure how to achieve the desired goal of making
this "just work" for our users.

[0]
http://git.savannah.gnu.org/cgit/guix/maintenance.git/tree/hydra/nginx/hydra.gnu.org-locations.conf#n14