Use large SaaS runner for compile-production-assets

What does this MR do and why?

After !164353 (merged) was reverted by !164406 (merged) due to gitlab-com/gl-infra/production#18468 (closed), this reapplies the change using an environment variable, which is supported by the tags keywords.

We now use the GITLAB_LARGE_RUNNER_OPTIONAL environment variable, which defaults to gitlab-org (which is available in dev!) that can be overwritten by the project variables.

• gitlab-org/gitlab>
  • using default runner: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/7704813254
  • using runner from project variable: https://gitlab.com/gitlab-org/gitlab/-/jobs/7704823326
• gitlab-org/gitlab-foss>
  • using default runner: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/7704782704
  • using runner from project variable: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/7704839342

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