Fix flakyness in isolation_data_migration.spec (#6122)
Fix flakyness in isolation_data_migration.spec (#6122)

The tests isolation_concurrent_dml and isolation_data_migration tests
were being run in parallel, but they were interfering with each others
output. Sometimes queries from isolation_concurrent_dml were blocking
create_distributed_table in isolation_data_migration:

1. https://app.circleci.com/pipelines/github/citusdata/citus/25562/workflows/f9d0a6ff-bb7a-4b71-9fcf-1a3e46d54425/jobs/713270
2. https://app.circleci.com/pipelines/github/citusdata/citus/25562/workflows/1e22454c-1623-48a7-97fb-c6803c7959c7/jobs/713223
3. https://app.circleci.com/pipelines/github/citusdata/citus/25562/workflows/618c419e-eefb-4582-9482-322dbb9ac96d/jobs/713110

This fixes it changing the schedule to not run these tests in parallel.

citusdata/citus,citusdata/citus,citusdata/citus,citusdata/citus,citusdata/citus