separate 'release' builds and non-release builds

[eyberg]

essentially we don't want to break anyone that might be using this in the future if master for whatever reason doesn't work - i don't know what we want to do here but some method of tagging a 'goodish' build would be good - for now we could manually cp in old builds into a release folder and use that and then generate each new build by the day and auto-delete after say 30 days or whatever

[tijoytom-zz]

https://console.cloud.google.com/storage/browser/uniboot/release/
Right now this need to be manually updated, but eventually we want to tie it to nanos release process.By default nvm will use the release bits, but you can force it to use latest dev binaries using --force flag