[CI] ci: generate badges from nightly job matrix

[jokasimr]

In our old multi-repo setup each badge on the ess-project page was associated with a single workflow run.

In the current setup that is not the case, for two reasons:

1. All three nightly tests are run in the same workflow (this could be changed).
2. Each package is tested in the same workflow (this could probably not be changed).

This means that if we want "per package" badges in the status page, we cannot use the /badge.svg url associated with the workflow, like we did before.

This PR implements a solution for this problem that lets us have a single nightly workflow for all nightly types and all packages, but it requires us to generate the badge svgs ourselves based on results retrieved from the github api in a separate step in the workflow.

It does sound a bit icky, but I think having per-project badges on the status page might be worth it.
What do you think? Is there a better solution?

Alternatives:

1. Split the nightly workflows per nightly-type (main, latest, lowest) and only display one badge per nightly-type, not one per package.
2. Create one separate workflow for each nightly-type / package combination.

[github-actions]

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