Reuse variables builder for Ci::Config rules evaluation

What does this MR do and why?

Related to #352587 (closed)

The initial plan was to couple only the secret variables under a single method in the variables builder, but I think this makes more sense.

The plan
Refactor instance secret variables	!79435 (merged)
Refactor project secret variables	!79850 (merged)
Refactor group secret variables	!79936 (merged)
Bundle them under a single method	!79935 (merged)	this one

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