Blender Git Loki

Archive build script: fix compatibility with older tar on CentOS 7

On CentOS 7, `tar --use-compress-program='xz -9'` tries to run `xz -9` as
executable, rather than running `xz` with `-9` as argument. Passing the
`-9` option via the `XZ_OPT` environment variable, as suggested by
@campbellbarton in D6138, works fine.