Use conditional assignment for BRANCH

Signed-off-by: Martin Schuppert <mschuppert@redhat.com>

Author: stuggi

Makefile

@@ -147,7 +147,7 @@ vet: gowork ## Run go vet against code.
 	go vet ./...
 	go vet ./apis/...
 
-BRANCH=main
+BRANCH ?= main
 .PHONY: force-bump
 force-bump: ## Force bump after tagging
 	for dep in $$(cat go.mod | grep openstack-k8s-operators | grep -vE -- 'indirect|openstack-operator|^replace' | awk '{print $$1}'); do \