Generate Bonsai.Setup's Bonsai.config from the same source as the portable zip

[PathogenDavid]

As discussed here #1873 (comment)

Right now the NuGet config for the installer and the portable zip comes from two separate places. It should come from a single source of truth.

Since Bonsai.Setup will mostly only ever be built by CI now, the easiest route would be to just generate it from the same Python script as create-portable-zip.py, or maybe use MSBuild instead to avoid the Python dependency on building Bonsai.Setup.