Make CI config fetch timeout configurable

What does this MR do and why?

This is a workaround for #43710 (closed). We introduced a timeout in !16807 (merged), which is hard-coded to 30 seconds. In some situations it is possible to exceed this timeout.

As a workaround, we make the timeout configurable by introducing the GITLAB_CI_CONFIG_FETCH_TIMEOUT_SECONDS environment variable.

This is intended for self-managed customers to be able to tune the values. We will keep the 30 second default on gitlab.com.

Screenshots or screen recordings

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How to set up and validate locally

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