devel/bmake: disable cmd-interrupt test in batch builds

This fixes the build under modern poudriere, but continues to run the test
in all other environments so that we *can* catch issues here if they arise.

The breakage should get fixed on the poudriere or base sh(1) side, but this
will not necessarily be done soon.

Reported by: mjg
Approved by: portmgr (implicit, bulk fix)
MFH: 2020Q4 (blanket: trivial build fix)