ci: extract the repeated smart_build call in the medium cascade

The package pre-build and the test job triggers were the same call with the
same package identity, spelled out twice. If the two ever drift apart the
test jobs stop finding the pre-built package and build their own, which is
exactly the thing the pre-build exists to avoid.

No behaviour change, the resulting build parameters are identical.

CMK-32643

Change-Id: I18f0a8d31c3e1c3333ec43214b4cad8f62288081