Ensure alt text is output when the avatar image is output

[dkotter]

Describe the bug

If an author has a local avatar set and the image used for that avatar has custom alt text set, it doesn't appear that we ever use that alt text (see this report). WordPress already supports outputting the alt text on the avatar image tag if that exists, so we just need to ensure that if someone has a local avatar set, let's grab the alt text for that image and return that in the image arguments.

Steps to Reproduce

1. Upload a new image and add alt text to it
2. Assign that image as a local avatar to a user
3. Create a post with that same user
4. View the post on the front-end and see the avatar being output but no alt text

Screenshots, screen recording, code snippet

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Environment information

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WordPress information

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