WorkspaceList.createTempDir (JENKINS-27152) is used by various plugins, and from Pipeline builds you can even use pwd tmp: true to use this location for custom purposes, but this is awkward. In practice many scripts wind up just binding this to a variable (example). We should define it out of the box.

$WORKSPACE is used for the main workspace (though not frequently needed as this is also the default working directory). $WORKSPACE_TMP seems like an intuitive name for the new property and matches the style of the @tmp suffix. Standard practice in Linux is to allow $TMPDIR to define a temporary directory, but using this variable name would result in a behavioral change for some existing scripts that may be unwanted, for example if /tmp is a RAM filesystem while the Jenkins workspace is persistent; we would like to offer a facility that users can opt in to where it makes sense (for example for project-specific caches) but not where it does not make sense (temporary files that are soon after deleted).