docker.io rate limits _should_ be addressed now. Switch back to
`debian-12` runners.
This is now a conditional within the `tests` workflow as we evaluate the
migration.
This reverts commit a99e2c62e6.
The self-hosted Github runners have been provisioned, and we can switch
to using them for evaluation.
To prefer Github-hosted runners, you can safely revert this commit.
See: t/123181.
