Adding repository to nuget publication

[tomap]

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 renovatebot/renovate#38220 )

Thank you,

Thomas Piart