release.sh: Pass ARCH_FLAGS when setting VMSIZE

[MFC note: This is not needed on stable/14 right now since the riscv64
images still fit within the default 5 GB size, but I want it in the
tree in case I need to bump VM sizes on a per-arch basis later.]

The default VM size may depend on the architecture. In particular,
it is currently larged on riscv64 due to a toolchain issue which
results in bloated binaries.

MFC after: 3 days
Fixes: 59c21ed6e811 "release: Bump default VM size for riscv64 to 6 GB"
Sponsored by: Amazon

(cherry picked from commit ed807f7bcaecbf32b0b81a7d18cee489a3666157)