release: Bump default VM size for riscv64 to 6 GB

Due to issues with the riscv64 toolchain, some binaries end up
significantly larger on riscv64 than they should be. This results
in riscv64 VM images -- and at present *only* riscv64 images -- not
fitting within the default 5 GB filesystem size.

Bump the default size for riscv64 to 6 GB until the toolchain issues
can be resolved.

MFC after: 1 week
Sponsored by: Amazon