Bump obuilder lower bound to 0.7.0 Qemu_sandbox and Hcs_sandbox (used by worker/obuilder_build.{ml,mli} and bin/worker.ml) only appear in obuilder >= 0.7.0; lower-bounds CI for v0.4.0 was still failing with 'Unbound module Obuilder.Qemu_sandbox' after the 0.6.0 bump.