Move CI to github actions

[r-ash]

In moving this to Avenir we'll no longer have access to the buildkite runners. We can use github actions free runners to achieve what we need instead.

This PR will

• Move CI build to github actions
• Remove branch pin on duckdb (they now have a release which is backwards and forwards compatible, so we can use this to read old results)
• Fix some R CMD check issues
• Fix vignette generation and rerun these