Might be… From the build’s log (the one you linked above):
Currently running NuGet.exe 4.3.0.
What’s the NuGet version you use locally?
Also, did you try to do a clean git clone, into a new directory, on your own Mac, and run NuGet restore etc. from there? Does it work from there? (related guide: How to debug your build locally / "It works on my Mac/PC but not on bitrise.io" )