Code review comment for ~info-martin-konrad/epics-base:fix-substitution-file-expansion

This one only fixes the immediate problem I ran into. https://code.launchpad.net/~info-martin-konrad/epics-base/+git/epics-base/+ref/clean-up-msi contains the same commits plus some additional clean up that should make similar problems less likely. I would recommend merging the latter.