Migrations can fail to run because cloud_sql_proxy is not ready

When migrations are run as part of a GitLab deployment, the migration script can fail because we don't wait long enough for cloud_sql_proxy to become available.

We've seen this problem twice in the Shib Metadata deploy project: https://gitlab.developers.cam.ac.uk/uis/devops/raven/shibboleth-metadata-deployment/-/jobs/236957.