Fix Makefile for Linux target when run on an Arm64 Mac

The k3sup binary is for Linux amd64, but was being created for
Linux arm64 and not Linux amd64.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Author: alexellis

Makefile

@@ -20,7 +20,7 @@ gofmt:
 dist:
 	mkdir -p bin/
 	rm -rf bin/k3sup*
-	CGO_ENABLED=0 GOOS=linux go build -ldflags $(LDFLAGS) -o bin/k3sup
+	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags $(LDFLAGS) -o bin/k3sup
 	GOARM=7 GOARCH=arm CGO_ENABLED=0 GOOS=linux go build -ldflags $(LDFLAGS) -o bin/k3sup-armhf
 	GOARCH=arm64 CGO_ENABLED=0 GOOS=linux go build -ldflags $(LDFLAGS) -o bin/k3sup-arm64
 	CGO_ENABLED=0 GOOS=darwin go build -ldflags $(LDFLAGS) -o bin/k3sup-darwin