Improvements for CI build artifacts

[etpinard]

I'm a big huge fan of @antoinerg 's #3136, so I couldn't resist improving it slightly.

This PR:

• makes the build artifacts get "published" even when the tests fail (which can be useful in some situations)
• echoes the artifact URLs in the publish step using Circle's built-in environment variables, which can serve as an easier click-to alternative to @antoinerg's fancy shell footwork.

cc @plotly/plotly_js

[antoinerg]

Thank you @etpinard :) I like your improvements as well 🎉

The only reason I had publish be the last build step was to have the following URL point to it:
https://circleci.com/api/v1.1/project/github/plotly/plotly.js/latest/artifacts?branch=master\&filter=successful. It turns out this was unreliable because latest could point to another job if the tests were still running. Therefore, it makes sense to run publish right after build!

💃