[tlbuild] Support for musl

[Norbert Preining]

> Protip:  You don't have to repeat before_script in both jobs.  Just move it outside the matrix.
> https://github.com/hmenke/texlive-source/commit/f37910f7e945d9eb3e6d72c9683709712e09b841

BTW thanks, included that in the main sources, named you in the travis
file, too ;-) Thanks a lot.

Norbert

--
PREINING Norbert                               http://www.preining.info
Accelia Inc.     +    JAIST     +    TeX Live     +    Debian Developer
GPG: 0x860CDC13   fp: F7D8 A928 26E3 16A1 9FA0 ACF0 6CAC A448 860C DC13