Make "upload codecov results" action a seperate action triggered by "test" action

[johannaengland]

We have figured out while working on Uninett/nav#2914 and Uninett/nav#2922 that when a workflow is triggered by another workflow it is run in a secure environment on the master branch and will have access to secrets of that repository.

This might be a good solution to our recent problem of codecov uploads failing since most of the developers work on their own forks, where the codecov action does not have access to the CODECOV_TOKEN and is therefore limited to tokenless uploading.

Which is why when there are many actions running at the same time will lead to the action failing since the limit for tokenless uploading is reached and it very annoying to have to run the action again.

[johannaengland]

Potentially relevant links:
https://docs.github.com/en/webhooks/webhook-events-and-payloads#workflow_run
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run