chore(ci): Trigger build when main is updated

[delanni]

Summary

Configures build on push to main (ie on-merge runs of the pipeline for when a PR is merged).

Details

The new pipeline is in place: https://buildkite.com/elastic/kibana-elastic-charts-build however, it's triggered currently by thehttps://github.com/elastic/buildkite-pr-bot. This won't trigger when the PR is merged to main, so we're adding another trigger to deal with pushes to main.

Issues

Relates to #2499

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• All related issues have been linked (i.e. closes #123, fixes #123)
• New public API exports have been added to packages/charts/src/index.ts
• Unit tests have been added or updated to match the most common scenarios
• The proper documentation and/or storybook story has been added or updated
• The code has been checked for cross-browser compatibility (Chrome, Firefox, Safari, Edge)
• Visual changes have been tested with light and dark themes

[markov00]

buildkite test this

[markov00]

buildkite test this

[delanni]

@markov00 @nickofthyme - it's probably the webhook configuration that's missing. I'm working on it!

Unless you would like to keep the datavis bot to trigger these jobs. If you do, please let me know where to find that bot, and I can try to adjust it.