pyrefly: ignore unbound-name in wscript_build files

These files declare `ctx: BuildContext` (or `bld`) without an
assignment because waf injects the value at evaluation time. Mypy and
ruff accept the bare annotation, but pyrefly flags every subsequent
reference as `unbound-name`. Add a `sub_config` override mirroring the
existing ruff `F821` per-file ignore.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Change-Id: I4fa934b056783a9458a5ad8f44c5eccfead4db8b
Priv-Id: d5d856bd088e99e034b1c123d082aace7630dff6

Author: nicopauss