The tags are named vX.Y, and we'll be 0.x for a while, so limiting the
publish-crate action to run v0 tags seems good enough to avoid
accidental publishes.
This introduces a new section of the GitHub Actions workflow which will
publish the crate upon a new tag being pushed. Note that this requires a
new project secret, `CRATES_TOKEN`.
This should fix the build failures due to unavailable components, e.g.
error: component 'rustfmt' for target 'x86_64-unknown-linux-gnu' is unavailable for download for channel nightly
Sometimes not all components are available in any given nightly.
