fix: makefile build needs to build in cmd dir

[lancebord]

When following build instructions in readme the following output is generated upon make build

CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -tags netgo -ldflags '-w -extldflags "-static"' -o apx
no Go files in /path/to/apx
make: *** [Makefile:12: apx] Error 1

The fix is to make it point to the cmd directory.