Add github actions jobs for producing macOS and windows artifacts

Github provides free windows and macOS hosts for their CI/CD offering called github actions (https://github.com/features/actions)

once this is merged and github mirror is updated, someone with access needs to enable the Github actions feature for https://github.com/leapcode/bitmask-vpn for this to start working and producing nightly builds.

currently the frequency for the job is set to run every sunday (i.e once a week)