[clang][ci] Improves buildkite artifacts.

The financial cost of the network I/O for the Clang install artifacts is
quite significant. afd3478f improved this by creating tarballs. This
commit improves the tarball by using xz compression instead of gzip. This
option is the slowest, but gives the smallest size.

      size  time           time
            (compression)  (decompression)
gzip  51 M  7  s           1.2 s
bz2   44 M  17 s           5.8 s
xz    33 M  76 s           3.1 s

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D150062