When a jobs is launched on a slave, ${WORKSPACE} is set to the slave root instead of the workspace directory when EnvInject is enabled (default settings in this case). I setup a simple example with a project that does

echo ${WORKSPACE}
pwd

The project is a basic project (named Test-Slave) - the only selection changed from the default is "Restrict where this project can be run" which I set to my slave. Here is the console for the job. You can see the difference between ${WORKSPACE} and pwd.

[EnvInject] - Loading node environment variables.
Building remotely on XL-build1 in workspace /home/scratch/jenkins_xl/workspace/Test-Slave
[Test-Slave] $ /bin/sh -xe /tmp/hudson4075547965590808000.sh
+ echo /home/scratch/jenkins_xl
/home/scratch/jenkins_xl
+ pwd
/home/scratch/jenkins_xl/workspace/Test-Slave