build(wheel): Take bison from a GNU mirror, ftp.gnu.org where that fails
The x86_64 wheel build of 275321c failed fetching bison: curl gave up
on ftp.gnu.org with "Failed to connect to 2001:470:142:3::b: Network
is unreachable", and CentOS 7's curl retries no failed connection, so
one unreachable host failed the build. It now asks ftpmirror.gnu.org,
which hands the request to a nearby mirror as GNU asks downloads to,
and falls back to ftp.gnu.org; the pinned SHA-256 still decides. Both
serve bison-3.8.2.tar.xz at that digest.