Sorry for the delay - I didn't notice your update.
I expect the WORKSPACE variable to contain the path value to the workspace directory for the current job.
Maybe the setup I used isn't that straight forward. Also I just realized that my testsetup is not what I expected. I missed the point that setting a custom workspace directory - using a relative path - gets appended to either JENKINS_HOME or the remote root directory for slaves.
Sorry for the troubles.
But one last point I almost missed is that setting environment variables using envinject ignores case sensitivity. Using the following example:
kills the second variable and only FLAGS will finally be set.