I created a new job and changed the workspace to a custom location by specifying
a different location. In my case I set the path to:
/tmp/hudsonBuilds/MyJob/${BUILD_ID}

Expected Result:
There will be a directory tree '/tmp/hudsonBuilds/MyJob/yyy-m-dd_hh-mm-ss'
created to reflect the current $BUILD_ID.

Current Result:
There is a directory tree which looks like
'/tmp/hudsonBuilds/MyJob/${BUILD_ID}'. The environment variable doesn't get
expanded.

The WORKSPACE environment variable instead has the correct path with expanded
BUILD_ID variable.