Allow nugetize (non-build) tool for dotnet-tool projects

[kzu]

For dotnet tool projects, the design-time build we perform fails since
the tool attempts to publish its output, which the project references won't
be able to supply and the whole thing fails with missing files.

We should pass IsPublishable=false to fake a sort of "DTB" for tools,
so they don't attempt to publish output.

Workaround for now is to invoke nugetize -p:IsPublishable=false