Use reusable workflow call instead of PRs for incremental builds

[Arlodotexe]

This PR is a follow-up to #696 which:

• Reverts to git-based merges instead of PRs
• Adds a reusable build workflow trigger that directly specifies the ref that build.yml should run on.

This is supported according to the docs at https://docs.github.com/en/actions/how-tos/reuse-automations/reuse-workflows#calling-a-reusable-workflow

[michael-hawker]

🤞