Add cake-addin tag to Cake.CoverageComparer NuGet package

The [NuGet Gallery](https://nuget.org) will soon include a "Cake" tab with instructions on how to use NuGet packages in Cake build scripts. I'd like to suggest that you include the tag `cake-addin` in the NuGet package of this addin, so that the NuGet Gallery can display the correct instructions to install this addin.

The UI will look similar to this:


Documentation on best practices on tags for Cake addins: https://cakebuild.net/docs/extending/addins/best-practices#tags

Inspect the repository's NuGet packaging metadata and compare it with the Cake add-in tag guidance linked in the issue. Verify the generated package metadata includes the `cake-addin` tag; the issue provides no specific file or test name, so first locate the package configuration used for publishing.

Written by the indexing model from the issue text.