build.sh:buildp_dispatch(): silence command(1) output.
    build.sh:build(): turn ${_build_date_start} into scoped global ${BUILD_DATE_START} due to vars/invariants_zipdist.vars.
    subr/ex_pkg_exec.subr:ex_pkg_exec(): correctly set ${_rc} from pkg_${_pkg_name}_all()'s return status.
    subr/ex_pkg_exec.subr:ex_pkg_exec(): only set `finish' package build state given zero ${_rc} and ${_restart_at}.