build: merge helm indexes instead of recreating for every release

A new helm repo index was created from scratch for every new release
which meant that the release timestamps in the index were basically all
identical.

This change intends to fix that by merging the new index with the
existing one

Fixes: rook#15604

Signed-off-by: Michael Adam <obnox@samba.org>

Author: obnoxxx

build/release/Makefile

@@ -121,7 +121,7 @@ promote.helm: $(HELM)
 #	copy existing charts to a temp dir, then combine with new charts, reindex, and upload
 	$(S3_SYNC) s3://$(HELM_S3_BUCKET)/$(HELM_CHANNEL) $(HELM_TEMP)
 	$(S3_SYNC) s3://$(S3_BUCKET)/build/$(BRANCH_NAME)/$(VERSION)/charts $(HELM_TEMP)
-	$(HELM) repo index --url $(HELM_URL) $(HELM_TEMP)
+	$(HELM) repo index --url $(HELM_URL) --merge $(HELM_TEMP)/index.yaml $(HELM_TEMP)
 	$(S3_SYNC_DEL) $(HELM_TEMP) s3://$(HELM_S3_BUCKET)/$(HELM_CHANNEL)
 	rm -fr $(HELM_TEMP)