nixpkgs: Give maintainers the ability to link issues and PRs

Currently committers have the ability to link issues and PRs, such that when the PR is merged, the issue is automatically closed. However, despite having the ability to manually close issues, regular maintainers cannot do this unless they control the original PR and use a closing keyword in the description.

This increases the burden of work for maintainers who triage incoming issues and link existing issues to PRs, as the issue requires either committer action or a manual follow-up to be closed. It also creates more noise, can't be filtered for, and is less clear at a glance.

If possible with enterprise permission controls, please grant maintainers this permission. (I couldn't figure out from what I can see publically whether this can be individually configured.) If not, this may be a good thing to bring up with GitHub as a low-priority item.

Start with GitHub's linked issue and pull request permission documentation, then inspect the organization-level controls available to maintainers. Verify whether this permission can be granted independently; if it cannot, record the limitation and the proposed GitHub follow-up as the completed outcome.