pkgbase: create package with SRCRELDATE not OSRELDATE as OSVERSION

In 188fe88ec50eac7c10e1d8350bf1180f8c16e463 OSVERSION has been set
to OSRELDATE which is the RELDATE of the building OS while we wanted
to use SRCRELDATE which is the RELDATE of the target system

(cherry picked from commit 051d69d6f8b51dc1d7bba58af99be76185312222)