[source-build] dotnet-deb-tool should not be restored if not creating Debian packages

[crummel]

dotnet-deb-tool is restored even when SkipBuildingInstallers is set to true, which causes the prebuilt dotnet-deb-tool package to be downloaded. Source-build currently works around this with this patch which is a pretty blunt-force approach, just removing the references to GenerateDebs. We should figure out a better way to do this.