build.yaml: Build shared branches

We're currently building every single commit pushed to Github. This is
racking up $20k in build charges a month and is generally superfluous.

This changeset alters build triggers so that builds automatically run if
they are committed to a **shared branch**:

- `release/*` - A release stabilization branch
- `project/*` - A project viewer branch
- `main/*` - The default/stable branch

PR commits are also automatically built.

...need to build another commit? Developers can trigger one using a
manual workflow run.

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