Adding repository to nuget publication

Hello,

I saw that https://www.nuget.org/packages/ExcelDna.Testing does not specify the source repository:

(via nuspec property or via SourceLink)

Unlike

This is super minor, but it'll prevent RenovateBot (and maybe dependabot) from nicely grouping nugets together when creating update Pull Requests

(related to https://github.com/renovatebot/renovate/pull/38220 )

Thank you,

Thomas Piart