Release multibuild binaries

[rburchell]

I can (and do) already multibuild with multibuild to test that things work. Might also be a good idea to get that done via GH actions for testing PRs, and longer term, stick them in a release.

[rburchell]

Might be multiple stages to this?

• Action to validate that go build and go test -v both pass at a minimum, on push and PR.
• Action to, on release (tag), build, run on itself, and upload binaries.

[rburchell]

Unfortunately, not so straightforward, see #15.

actions/create-release is deprecated, so I see no easy way to push releases, so there's nowhere to stick the binaries for now.