Fix Poetry caching for accessibility tests

[wren]

The cache key was changed a while back, and we left out the accessibility tests. The tests still ran, but no longer cached the installed packages through poetry, resulting in slower tests. This PR fixes it, and should speed up the accessibility tests.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.