Add github action to publish release artifacts

[trown]

This adds an action which runs when a "v*" tag is pushed to the
repo. This action does a basic release build on ubuntu-latest and
then creates a release and uploads the release binary to that
release.

[trown]

This is for #14

Doing git tag v0.1.1 && git push origin v0.1.1 gives: https://github.com/trown/coretoolbox/releases/tag/v0.1.1

This could be easily expanded to build a few release binaries if there is demand for some particular arch.

Tangentially, I noticed they fixed artifact caching for rust recently: actions/cache#133 (comment). I could add that to both of the rust CIs.