release.sh: Pass ARCH_FLAGS when setting VMSIZE

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