fix: fall back to a second go proxy on any download error

goproxy.cn intermittently returns HTTP/2 stream errors while fetching
module zips (seen with github.com/dokku/docker-port-forward), which the
default comma-separated list does not fall back from since it only
retries on 404/410. Use the pipe separator so any error falls through to
the aliyun mirror and then direct.

common.mk

@@ -22,7 +22,7 @@ build-in-docker: clean
 		-v $(GO_MOD_CACHE):/go/pkg/mod \
 		-e PLUGIN_NAME=$(PLUGIN_NAME) \
 		-e GO111MODULE=on \
-		-e GOPROXY=https://goproxy.cn,direct \
+		-e "GOPROXY=https://goproxy.cn|https://mirrors.aliyun.com/goproxy/|direct" \
 		-w $(GO_REPO_ROOT)/plugins/$(PLUGIN_NAME) \
 		$(BUILD_IMAGE) \
 		bash -c "GO_ARGS='$(GO_ARGS)' CGO_ENABLED=0 GOOS=linux GOARCH=$(GOARCH) GOWORK=off make -j4 $(GO_PLUGIN_MAKE_TARGET)" || exit $$?