actions: automate release build with github actions

[philips]

See https://github.com/transparencylog/tl/pull/20/commits/0e35e0d3feaf9c8c42e1e0bbff8acb9a979c08db

Right now I would prefer to babysit the release process. But, it is probably a good idea in the future if there are more contributors.

[philips]

cc @rothgar

[rothgar]

That's totally reasonable.

Do you want goreleaser to build the deb and rpm images too? I can send a PR to update goreleaser to use nfpm https://goreleaser.com/customization/nfpm/