Fix broken FOSS master `graphql-docs-verify` job

What does this MR do?

Skips the gitlab:graphql:check_docs task unless run in EE.

Fixes a master:broken for gitlab-foss #34293 (closed).

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Closes #34293 (closed)