Have Cirrus CI run cargo release --execute if version = ... was changed

[duckinator]

cargo-dist can be set up so triggering the "Release" GitHub Workflow with the appropriate tag will create everything.

The tag used should be v followed by the version specified in Cargo.toml.

Cirrus CI tasks can be configured to only run when a diff includes a specific file.

It should be possible to check if the version was changed and, if so, trigger the "Release" workflow on GitHub. (This will likely need to be a runtime check -- if it's changed, trigger Release; otherwise do nothing and succeed. I don't think Cirrus' only_if/skip filters are advanced enough for this.)