For pg_upgrade's test we (unless prevented by the caller via via
NO_TEMP_INSTALL) built a separate installation. That causes an
unnecessary slowdown after the infrastructure introduced by
dcae5fac (and unnecessarily duplicates code).

Author: Andres Freund
Reviewed-By: Tom Lane
Discussion:
    https://postgr.es/m/20190521191918.z7kwnrlj45mk2k67@alap3.anarazel.de
    https://postgr.es/m/20190521195209.qfzwfxvymguuwlu5@alap3.anarazel.de