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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20. PR:
21. Reported by: <If someone else reported the issue.>
22. Reviewed by: <If someone else reviewed your modification.>
23. Approved by: <If you needed approval for this commit.>
24. Obtained from: <If the change is from a third party.>
25. MFC after: <N [day[s]|week[s]|month[s]]. Request a reminder email>
26. MFH: <Ports tree branch name. Request approval for merge.>
27. Relnotes: <Set to 'yes' for mention in release notes.>
28. Security: <Vulnerability reference (one per line) or description.>
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30. Pull Request: https://github.com/freebsd/<repo/pull/###>
31. Differential Revision: https://reviews.freebsd.org/D### #
32. "Pull Request" and "Differential Revision" require the *full* GitHub or
33. Phabricator URL. The commit author should be set appropriately, using
34. git commit --author if someone besides the committer sent in the change. #