feat: templated cache secret

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What does this MR do?

This MR templates cache secret (.Values.runners.cache.secretName) for gitlab-runner (tpl function)

Why was this MR needed?

It seemed odd that runner secret was already templated (.Values.runners.secret), but not the cache one.

What's the best way to test this MR?

Just add tpl function for .Values.runners.cache.secretName

What are the relevant issue numbers?