Set up releases via Github Actions about cvcap

[crispinb]

Essentially done. Remaining issues:

• Github runner is on rust 1.62.1. We need 1.63 (for scoped threads). If not updated soon, stash the scoped threads commit for now
• name release manually rather than from tagged commit (probably a softprops/action-gh-release setting)
• can we stop action-gh-release from adding the tagged commit body as the release notes?

musl build doesn't work. Deps or linking issue I think. Separate issue: #16

[crispinb]

PENDING github runner updating Rust to 1.63 (they do have weekly releases)

[crispinb]

Abandoning for now as the download count is so tiny I think it's only me. And the GHA workflow breaks every release & is a nightmare to debug, and is generally a huge time waster.