Run detect-tests job when only rubocop files are changed

Context

See the result of this investigation:

The rubocop job doesn't really need predictive tests, but it needs the list of files that were changed in the MR, which is something the detect-tests job will give as a by-product.

An easy fix is to make sure the detect-tests CI job will run when we modify the .rubocop_todo folder, which is what I'm proposing in !151660 (merged).

Closes #459862 (closed).

What does this MR do and why?

Run the detect-tests job if we only change rubocop files.

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