test-sysroot: Skip on s390x by default
This test regularly fails on the buildds, but I cannot reproduce the
failure on a porterbox.
Bug: https://github.com/ostreedev/ostree/issues/2527
Bug-Debian: https://bugs.debian.org/
1025532
Forwarded: not-needed
Gbp-Pq: Topic debian
Gbp-Pq: Name test-sysroot-Skip-on-s390x-by-default.patch