Default build-image target arch to amd64 (grafana#11148)

**What this PR does / why we need it**:

The changes in grafana#11130 broke `make
build-image` and `make build-image-push`. This defaults the target arch
added in grafana#11130 so those make targets still work.

Makefile

@@ -101,15 +101,15 @@ RM := --rm
 # in any custom cloudbuild.yaml files
 TTY := --tty
 
-DOCKER_BUILDKIT=1
+DOCKER_BUILDKIT ?= 1
 BUILD_IMAGE = BUILD_IMAGE=$(IMAGE_PREFIX)/loki-build-image:$(BUILD_IMAGE_VERSION)
 PUSH_OCI=docker push
 TAG_OCI=docker tag
 ifeq ($(CI), true)
 	OCI_PLATFORMS=--platform=linux/amd64,linux/arm64
-	BUILD_OCI=docker buildx build $(OCI_PLATFORMS) --build-arg $(BUILD_IMAGE)
+	BUILD_OCI=DOCKER_BUILDKIT=$(DOCKER_BUILDKIT) docker buildx build $(OCI_PLATFORMS) --build-arg $(BUILD_IMAGE)
 else
-	BUILD_OCI=docker build --build-arg $(BUILD_IMAGE)
+	BUILD_OCI=DOCKER_BUILDKIT=$(DOCKER_BUILDKIT) docker build --build-arg $(BUILD_IMAGE)
 endif
 
 binfmt: