Extra files in build container

[Gregor Giesen]

Hello Guix,

I'm trying to pack unbound, however its unittests require /etc/protocols
and /etc/services. Now, I have found them in the net-base package 
which I included in native-inputs, but that is not sufficient for
glibc's getprotoent and getservent to find them. Can I symlink the
net-base ones to the build container's /etc? Adding an extra phase 
before 'check does not allow me to write /etc in the container.

Cheers,
Gregor