Move population of QA_TESTS var into a separate job

What does this MR do and why?

Describe in detail what your merge request does and why.

As part of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1287, we need QA_TESTS to determine if package-and-qa should be run. For this purpose, this MR moves the population of QA_TESTS to a separate job in prepare stage.

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