Use at least Power9 builders for c10s ppc64le builds

[thozza]

We have been seeing consistent failures in RPM builds for c10s ppc64le in COPR. E.g. https://download.copr.fedorainfracloud.org/results/@osbuild/osbuild/centos-stream-10-ppc64le/07697854-osbuild/builder-live.log.gz
The log contains multiple instances of signal 4 errors, which is SIGILL (illegal instruction).

COPR uses Power8 for builders, which is too old even for c9s.

Please use at least Power9 builders for c10s.

[praiskup]

Thank you for the report!

coprdb=# update mock_chroot set tags_raw='arch_power9' where id = 206;
UPDATE 1