metadata: refactor _builds_to_yaml to use dicts and _format functions

_builds_to_yaml does not use any features of the metadata.Build class, so
it can operate on plain dicts as well.  It also does not need to output
Build instances because those are converted to plain dicts when writing out
to YAML.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f