zlib: Fix zlib.net URL. (#21753)

Look for primary source file below
  http://zlib.net/fossils/ as opposed to
  http://zlib.net/
. zlib-1.2.8.tar.gz is still available at the former location, and will likely
remain there. In addition, it's important that the first URL work since zlib
is in the bootstrap path, and 16.09 (at least) bootstrap doesn't try to fetch
from later ones.