build/circleci: update to Ubuntu 20.04 image for release-* jobs
build/circleci: update to Ubuntu 20.04 image for release-* jobs

Background: `machine: true` is deprecated:

    https://circleci.com/docs/2.0/images/linux-vm/14.04-to-20.04-migration/

zrepl/zrepl,zrepl/zrepl