ci: Fix config for rspec-ee-pg12-rerun-previous-failed-tests

What does this MR do and why?

While documenting !69053 (merged), I noticed that rspec rspec-ee-pg12-rerun-previous-failed-tests wouldn't pull the pre-requisite artifacts from detect-previous-failed-tests due to a CI misconfiguration.

The problem was that extending from .rspec-ee-base-pg12 would override the previous extends, so that the needs definition was incorrect and wouldn't pull the detect-previous-failed-tests job's artifacts.

Example of the problem can be seen at https://gitlab.com/gitlab-org/gitlab/-/jobs/3390598302:

$ rspec_rerun_previous_failed_tests tmp/previous_failed_tests/rspec_ee_failed_files.txt
cat: tmp/previous_failed_tests/rspec_ee_failed_files.txt: No such file or directory
wc: tmp/previous_failed_tests/rspec_ee_failed_files.txt: No such file or directory
No failed test files to rerun

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