widened_labels: derive the xl nightly slot's std label like the root spill
Fixes #1820 (follow-up from the review of #1819).
widened_labels() added std's root label to a registered xl root runner (#1817) but not std's nightly label to a registered xl nightly slot, so a slot registered before #1819 got glaeda-nightly-std-xcode-<v> only when reinstalled. It now widens nightly slot labels by nightly_classes() the same way (xl -> xl, std; never light), through a new nightly_label() (the label member_labels() issues). Pure and idempotent as before; std and light slot labels and glaeda-nightly-only-m4pro are unchanged.
Tests: scripts/test-glaeda-cmux-runner.py FleetLabelsModuleTest ManifestLabelsTest OK, with a registered xl slot converging and the unchanged cases.
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