make: fix MAKE_JOB_ERROR_TOKEN

The rework of GetBooleanVar to GetBooleanExpr requires
we add "${" and ":U}" around the expression so it can be directly
evaluated.

Reported by: mjg
MFC after: 1 week

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(cherry picked from commit 3b96abbab03327176b1e4ee02a6742bf9807dd75)