Add link to the cyhy-system repository

Issues for this project should be infrastructure- related only. This adds a
link to the cisagov/cyhy-system issues page to encourage CyHy environment
issues to be created there instead.

.github/ISSUE_TEMPLATE/config.yml

@@ -1,5 +1,8 @@
 blank_issues_enabled: false
 contact_links:
+  - name: Submit an issue for the CyHy environment.
+    url: https://github.com/cisagov/cyhy-system/issues
+    about: Non-infrastructure issues should be made in the linked repository.
   - name: 💬 Questions & Help
     url: https://github.com/cisagov/.github/blob/develop/QUESTIONS.md
     about: If you have questions, visit GitHub Discussions and Wiki pages