Solution won't build if Nuget package path is different from default

`generate_protos.bat` defaults the NugetPath to `set NUGET_PATH=%UserProfile%\.nuget\packages`. If you have the environment variable `%NUGET_PACKAGES%` set, then nuget packages are written to that directory instead. Without the GRPC tools in `set NUGET_PATH=%UserProfile%\.nuget\packages`, the protos won't be built and the host will not finish building.

This could be fixed by using `%NUGET_PACKAGES%` as `NUGET_PATH` if the environment variable is set.

#### Repro steps

1. Delete the GRPC tools at %UserProfile%\.nuget\packages\google.protobuf.tools\3.5.1\tools
2. Change `%NUGET_PACKAGES%` to some other directory
3. Perform a clean build

#### Known workarounds

`%NUGET_PACKAGES%\google.protobuf.tools\3.5.1\tools` to `%UserProfile%\.nuget\packages\google.protobuf.tools\3.5.1\tools` should work.