Add macos build support (tested on M2)

[Schoonmoeder]

I managed to build it on macos (M2) but I needed to change the Build command slightly. please add / test following code in the make file

build-macos:
	@echo "Compiling source"
	@mkdir -p build
	env GOOS=darwin GOARCH=amd64 go build $(GO_EXTRA_BUILD_ARGS) -ldflags "-s -w -X main.version=$(VERSION)" -o build/chirpstack-simulator cmd/chirpstack-simulator/main.go

run docker-compose run --rm chirpstack-simulator make clean build-macos

now you are able to start it on mac and also create configs like in the readme.