Build priority 800 for mysql-9.7 on riscv64

[Sebastian Ramacher]

Hi Adrian,

we decided to kill the build of mysql-9.7 on riscv64. It is not
migrating to testing (#1142013), so having it built is not important. Please 
avoid
such bumps while the queue on riscv64 is that long.

Cheers
-- 
Sebastian Ramacher