Make PLATFORMS overridable for single-arch test builds

Default still builds linux/arm64/v8+linux/amd64 for shipping,
but 'make PLATFORMS=linux/amd64' skips the arm64 leg on hosts
without qemu-user-static registered.

Makefile

@@ -20,10 +20,11 @@ scripts/embassy.js: $(TS_FILES)
 	deno run --allow-read --allow-write --allow-env --allow-net scripts/bundle.ts
 
 KAMADO_SRC ?= ../KamadoPool
+PLATFORMS ?= linux/arm64/v8,linux/amd64
 
 image.tar: Dockerfile docker_entrypoint.sh
 	docker buildx build --tag start9/$(PKG_ID)/main:$(PKG_VERSION) \
-		--platform=linux/arm64/v8,linux/amd64 \
+		--platform=$(PLATFORMS) \
 		--build-arg ARCH=$(shell uname -m) \
 		--build-context kamado=$(KAMADO_SRC) \
 		-o type=oci,dest=image.tar .