ci: use the right docker image version in published artillery versions

[bernardobridge]

Description

Creates script to replace DEFAULT_IMAGE_TAG dynamically, and makes some changes to workflows to support it:

• 1be8638: starts running docker publish as part of the npm publish. This is to prevent race condition where npm build might be pushed (with tests starting to use it) before publish is done.
• 7d6e8c9, 7e18103: add replace script and use it in canary/artillery publishes
• 219bff2: also publish worker image on canary publishes, since it is no longer doing so with push:main trigger
• dbe2c01: only run canary when not releasing. this prevents images being published twice with the same tag, but also just makes sense, and prevents what currently happens where every release has a canary version published with it, which is unnecessary and confusing

Testing

Tested the substitution locally with a couple of strings and variations on the file. The canary that gets published to main on merging this should also test it!

Pre-merge checklist

This is for use by the Artillery team. Please leave this in if you're contributing to Artillery.

• Does this require an update to the docs? No
• Does this require a changelog entry? Maybe, it does prevent the bug we had with last release.