Move the CLUSTERGROUP env variable for previews inside its section

This way the code is a bit less confusing and it's more obvious
when/where CLUSTERGROUP is used.

make preview-% still keeps working as usual and you can override things
via `make CLUSTERGROUP=group-one preview-hello-world`

Gotta love Makefile's idiosyncrasies around per-target variables

Author: mbaldessari

Makefile

@@ -1,5 +1,4 @@
 NAME ?= $(shell basename "`pwd`")
-CLUSTERGROUP ?= $(shell yq ".main.clusterGroupName" values-global.yaml)
 
 ifneq ($(origin TARGET_SITE), undefined)
   TARGET_SITE_OPT=--set main.clusterGroupName=$(TARGET_SITE)
@@ -62,6 +61,7 @@ preview-all: ## (EXPERIMENTAL) Previews all applications on hub and managed clus
 	@common/scripts/preview-all.sh $(TARGET_REPO) $(TARGET_BRANCH)
 
 preview-%:
+	$(eval CLUSTERGROUP ?= $(shell yq ".main.clusterGroupName" values-global.yaml))
 	@common/scripts/preview.sh $(CLUSTERGROUP) $* $(TARGET_REPO) $(TARGET_BRANCH)
 
 .PHONY: operator-deploy