E3-894: Fix issue where non-existent versions would cause the build to hang

The ultimate issue here was that we performed a

cat $( ... ) | sed '1d'

and if that inner block was empty, then cat reverted to waiting for user input. The error also came up with non-specified build numbers, so we need to make sure that those are also being fetched correctly.

The upshot is now <module>_VERSION will automatically fetch the correct build number; the only potential downside is that if we have a newer version in ${E3_SITEMODS_PATH} in comparison to ${EPICS_MODULES} (read: cellMods), then it won't pick that up. However, this seems to be a pretty special corner case.