Create sync index for pipeline_id_convert_to_bigint

What does this MR do and why?

As titled, this is the second step of creating indexes asynchronously. See guideline at https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#schedule-the-index-to-be-created

It's to create the index synchronously for ci_pipeline_variables.pipeline_id_convert_to_bigint after the async index verification. So that we can start to prepare the foreign key for pipeline_id_convert_to_bigint as next step since the index is in place.

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Related to #412864 (closed)