Long term plan is still to do JuliaPackaging#778, but that's a lot of work, and we have little
time at the moment.  Simply splitting the logs into a separate tarball, instead,
is a simpler short-term solution which has some very important bonuses:

- the git tree hash of the main tarball should be reproducible (as long as you
use the same toolchain)
- which means we can start testing reproducibility of builds
- rebuilding a package without changing its content won't create a new artifact:
  less strain on the Package Storage Server.