On Freestyle jobs defining a Jenkins global variable like this would work as expected:

XDG_CACHE_HOME=${HOME}/.cache/${EXECUTOR_NUMBER}

Note: behavior is the same regardless if brackets are used or not when you define the variables.

Freestyle "set" result (correct):

XDG_CACHE_HOME=/home/builder/.cache/2

Pipeline "set" result (incorrect):

XDG_CACHE_HOME=${HOME}/.cache/${EXECUTOR_NUMBER}

This problem is essential because it prevents Jenkins administrators from using environment variables to fix/workaround execution problems. 

Effectively this breaks any job that needs to define variables that are relative to something else, like workspace, home, ....