Fix failing CI tests

[GenevieveBuckley]

The CI tests are currently failing.

Problem description

See here for an example: https://github.com/napari/napari-animation/actions/runs/9218650940

The reason for this failure is that imageio-ffmpeg does not include the FFMPEG binary for Apple Silicon Macs (see issue here imageio/imageio-ffmpeg#71). Since the github actions runners recently switched over so that macos-latest runs on Apple Silicon. That means we now get an error that imageio-ffmpeg cannot find the FFMPEG executable.

Proposed solution

Update the github actions workflow to check if it is running on Mac, and if it is then brew install ffmpeg (with this github action. We can link to imageio/imageio-ffmpeg#71, and note this is a temporary workaround that can be removed when imageio-ffmpeg releases a fix.