Using Declarative Pipeline set environment:

    environment {

                TEST="$WORKSPACE/test"

                ...

                SPEC='linux_x86-64'

                }

works fine on linux.

On Windows 
WORKSPACE=C:\Users\jenkins\workspace\openjdk_test_x86-64_windows
TEST will be C:Usersjenkinsworkspaceopenjdk_test_x86-64_windows/test. That is saying passing $WORKSPACE to toString() return discard all backslashes.

Tried using slash string TEST=/$WORKSPACE/ +'/test' got same issue.