workflows/tests: use Docker image for Ubuntu 20.04

[p-linnane]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
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• Have you successfully run brew tests with your changes locally?

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Brownouts have started for Ubuntu 20.04. Let's use our Docker image instead.

[p-linnane]

Alright with @Bo98's help we've got this working as expected. Updated the commit message to reflect the new approach.