ci: Don't use dependency proxy for community contribution MRs

What does this MR do and why?

This fixes #378175 (closed) by disabling the dependency proxy if the MR has Community contribution so that pipelines started by wider community contributors in the canonical context (I believe this happens for a push by the contributor after a first pipeline was triggered by a team member) don't fail because the dependency proxy isn't accessible to the contributor.

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.

Related to #378175 (closed)