Stop using /opt/pulumi/nuget, this makes some progress towards fixing
There are three main changes here:
1) We no longer pack on build, instead requiring an explicit pack command
(which runs in relase configuration). This should be a small speed improvement
for standard builds.
2) The integration tests no longer rely on a nuget package, instead we just
reference the project directly. This removes the need for a local nuget feed
that had to be setup and installed into.
3) As we've got rid of the local nuget feed `make install` simply runs "dotnet
pack" and `make publish` picks up the .nupkg from the sdk bin folder.
The one workflow this changes is if people have pulumi programs outside
pulumi/pulumi that they we're using `make install` to get a build of the
dotnet sdk into. Anyone using that workflow instead needs to either:
1) Manually copy the .nupkg from the build folder to wherever they have
setup their local nuget feed.
2) Change that project to refer to Pulumi via a project reference.
Fix the following warning and turn on "treat warnings as errors" for our .NET projects.
```
Deployment/InvokeOptions.cs(6,60): warning CS1574: XML comment has cref attribute 'InvokeAsync{T}(string,
```
