Include comparison URLs in PR body

[dawidd6]

Description

I think it would be useful to include links to GitHub (for now) comparison pages when flake inputs get updated.

Test PR: https://github.com/dawidd6/nix/pull/31

Checklist

• [x] Tested functionality against a test repository (see "How to test changes")
• [ ] Added or updated relevant documentation (leave unchecked if not applicable)

[dawidd6]

Could anyone please take a look at this PR? I'd like this functionality upstream.