[update] Clarify messaging around which runners are updating

[tsibley]

nextstrain update predates runners other than Docker and its messaging assumes that Docker is the runner in use. This isn't always the case anymore, and the output of update can be confusing as it's a mix of messages the CLI prints directly as well as output from docker image pull.

The update command could either operate on all supported runners (like nextstrain check-setup) or only operate on the default runner. In either case, runners other than Docker would want to print that they don't currently support updating. (In the future they might.)

• Remove the || true and associated todo comment in our CI workflow when this issue is fixed:

  cli/.github/workflows/ci.yaml

  Lines 72 to 74 in 4a95c2b

  	# XXX TODO: Stop ignoring errors once `update` is improved. See
  	# <https://github.com/nextstrain/cli/issues/87>.
  	nextstrain update || true