use the existing key to sign binaries when build with hack/release.sh

@tianon, I moved the ``make`` into a function - it took me a while to realise that main() was not actually the code that did the work...

I also feel uneasy about the ``gpg --gen-key --batch`` I'd rather the release process fall over in a screaming mimi, and demand that the person running it set things up 

Issue #4675