Update variable ordering in Makefile

PGO_VERSION was declared after its first usage. While this may
not have affected people who have the variables predefined in
their environment, this makes it a bit easier to build.

Author: psavva

Makefile

@@ -5,8 +5,8 @@ PGOROOT ?= $(CURDIR)
 PGO_BASEOS ?= centos8
 BASE_IMAGE_OS ?= $(PGO_BASEOS)
 PGO_IMAGE_PREFIX ?= crunchydata
-PGO_IMAGE_TAG ?= $(PGO_BASEOS)-$(PGO_VERSION)
 PGO_VERSION ?= 4.7.0
+PGO_IMAGE_TAG ?= $(PGO_BASEOS)-$(PGO_VERSION)
 PGO_PG_VERSION ?= 13
 PGO_PG_FULLVERSION ?= 13.3
 PGO_BACKREST_VERSION ?= 2.31