CI: setup QEMU to clobber unused vector elements for RVV

When mask-agnostic and tail-agnostic mode is set, QEMU's default behaviour is to still act like the mode is set to tail-undisturbed and mask-undisturbed, so a programmer could forget to set the mode correctly when using masks or differing vector sizes, and CI will still pass in some cases.

These QEMU_CPU flags toggle the behaviour that -agnostic modes should be treated as if they always destroy data, so it will more likely detect these problems, although QEMU might not respect this fully for all operations.