Fix the `migration paths` CI job

## What does this MR do?

It fixes the `migration paths` CI job. This job started failing at the time we added a migration that is actually using Redis.

The issue was that the `config/resque.yml` file we're using is not back-compatible with the `v8.5.9` version we're checking out in the first part of the job, that's why we're using a UNIX socket for this version.

To fix the issue:

* We use 'SETUP_DB: 'false'` in the job variables to speed up the job (it does not make sense to run migration before checking out v8.5.9)
* We just copy config/resque.yml.example to config/resque.yml at is simply has correct structure when checked out from v8.5.9
* When we revert to CI_BUILD_REF, we run prepare script again, which swaps config/resque.yml again, using new version this time.

Fixes #23658.

Note: the `migration paths` CI job is run on `master` only but you can see a successful build here: https://gitlab.com/gitlab-org/gitlab-ce/commit/fd99ec1a8ecc49ef70c4358ca7be2987f2a7505f

See merge request !7092