MB-7455: Fix ep-engine version to allow override for releases

The script was broken that allowed us to override the normal
git version that is used automatically. This change is
neccessary for production builds to reduce confusion when
looking at stats.

Change-Id: I9bc893beaa9f9ab057a0a8dd300933358e1acccb
Reviewed-on: http://review.couchbase.org/23664
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>