ci: Run 'detect-tests' in MRs only

Rémy Coutablerequested to merge
375245-dependency-on-detect-tests-should-be-optional-as-it-s-not-required-outside-of-mrs into master

What does this MR do and why?

Given than detect-tests only provides meaningful artifacts in MR pipelines, we should make the dependency on it optional for .rspec-base and change .rails:rules:detect-tests to only run for MRs.

This is a small optimization.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #375245 (closed)

Edited by Rémy Coutable

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