I thing there is a bug, when try to specified relative path to Nuget.exe.  I have multiple projects building on jenkins slave. I will like to use relative path like this: 
${WORKSPACE}\tools\NUget\NuGet.exe

ConsoleOutup: Caused: java.io.IOException: Cannot run program "${WORKSPACE}\tools\NUget\NuGet.exe" (in directory "C:\jenkins\workspace\PublishTest"): CreateProcess error=2, The system cannot find the file specified
Like u see in output, ${WORKSPACE} is not resolved, but OK. 
(in directory "C:\jenkins\workspace\PublishTest")
This is the working directory of the project therefore path like tools/NUget/Nuget.exe should work. But dont! 
Caused: java.io.IOException: Cannot run program "tools\NUget\NuGet.exe" (in directory "C:\jenkins\workspace\PublishTest"): CreateProcess error=2, The system cannot find the file specified
I used Process Monitor program from sys internal to find out where jenkins search for Nuget.exe. I found out that jenkins never searched in working directory, but in C: root directory instead. 

Temporary workaround in my case is, that i moved Nuget.exe outside of jenkins installation path, and use absolute path to Nuget.exe instead of relative.