Increase the size of riscv GENERICSD images to 6 GB

The stable/13 snapshot this week failed to build the riscv GENERICSD
image because it ran out of space. Checking main and stable/14
snapshots, they are also low on space, around 100% or more of
capacity. Increase them all from 5 GB to 6 GB. Note, this is the
only riscv image configuration.

Discussed with: cperciva