Add cake-addin tag to Cake.Debootstrap 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 project and NuGet packaging configuration to find where package tags are defined, then review the linked Cake add-in tagging guidance. Add the requested cake-addin metadata and verify the packed NuGet package contains the tag.

Written by the indexing model from the issue text.