Avoid duplicating user selection logic in features

Multiple scripts have similar / the same logic for determining which user should be selected for feature installation. We need to unify this to avoid duplication and annoying issues down the line.
https://github.com/devcontainers/features/pull/1530/changes#diff-26d9422c201004aa5880edd23a05e8e5cd69c42a2395391dd23b4ab2a62aa26bR246-R264