fix docker build args value, as this passes in a literal double single quote

Author: slmagus

Makefile

@@ -38,7 +38,7 @@ GO_LDFLAGS += '
 EXECUTOR_PACKAGE = $(REPOPATH)/cmd/executor
 WARMER_PACKAGE = $(REPOPATH)/cmd/warmer
 KANIKO_PROJECT = $(REPOPATH)/kaniko
-BUILD_ARG ?= '' 
+BUILD_ARG ?= 
 
 out/executor: $(GO_FILES)
 	GOARCH=$(GOARCH) GOOS=linux CGO_ENABLED=0 go build -ldflags $(GO_LDFLAGS) -o $@ $(EXECUTOR_PACKAGE)