Pass GITLAB_METRICS_EXPORTER_VERSION to CNG-mirror pipeline

What does this MR do and why?

GITLAB_METRICS_EXPORTER_VERSION is a file at the root of the project.

This variable is added to the build.env file, but we do not pass it to CNG-mirror pipelines (we will then use the main branch in CNG-build as the source for the container code).

This MR passes the GITLAB_METRICS_EXPORTER_VERSION environment variable to CNG-mirror pipelines.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.