The automated tests running in CircleCI don't actually use the `zulip`
db, so we can skip running migrations on it in some CircleCI shards to
save time.
NOTE: This only effects build jobs that run provision, except the
`production-build` job where we skip building the dbs altogether.
Migrations still run on `focal-backend` build job to ensure
we are testing all our development setup code.
/bin/sh and /usr/bin/env are the only two binaries that NixOS provides
at a fixed path (outside a buildFHSUserEnv sandbox).
This discussion was split from #11004.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>