Disable ffi on cross builds.
Disable ffi on cross builds.

Not really needed (don't have a cross python to run) and avoids
the unfortunate random test failures.

randombit/botan,randombit/botan,randombit/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,webmaster128/botan,randombit/botan