CI: GCC no longer publishes .bz2 tarballs

[Johannes Pfau via D.gnu]

I just updated the gdc-6 branch in my repo to gcc 6.4.0 and the CI
started to fail downloading the tarball. Turns out that the .bz2
tarball was replaced with a .tar.xz tarball for gcc 6.4.0 and I guess
it'll be similar for all new releases:
ftp://ftp.mirrorservice.org/sites/sourceware.org/pub/gcc/releases/gcc-6.4.0/

What's the best workaround for this? Simply trying .xz first and if
that fails trying .bz2? Or switching to .gz tarballs entirely?

-- Johannes