build-lectures: `-n` (nitpick) absent from default strictness; no extra-args passthrough via build-jupyter-cache

[mmcky]

Surfaced by the v0.8.0 validation study comparing the composite suite against the hand-rolled native pipelines (lecture-python-intro / lecture-python-programming).

Problem

build-lectures defaults extra-args to -W --keep-going (build-lectures/action.yml:25) — it does not include -n (nitpick), which the native intro/programming builds pass. Cross-reference warnings that fail a native build can therefore pass silently on the composite.

Compounding this, build-jupyter-cache invokes build-lectures for the HTML build with a fixed with: block and no extra-args passthrough (build-jupyter-cache/action.yml, the "Build HTML" step). A consumer of build-jupyter-cache cannot get -n — or any extra flag — into that HTML build at all.

Why it matters

A repo migrating from the native pipeline to the composite can silently lose nitpick strictness, so previously-failing broken cross-references start passing CI.

Suggested fix

• Decide and document a standard build strictness for the org (native is itself inconsistent on -n, so standardize deliberately).
• Add an extra-args / html-extra-args passthrough to build-jupyter-cache so strictness flags reach the inner build-lectures HTML build.
• Document the native-equivalent invocation (extra-args: '-n -W --keep-going') in the migration guide.

Direct build-lectures consumers can already pass extra-args: '-n -W --keep-going'; this issue is about the default and the build-jupyter-cache passthrough gap.

Context: QuantEcon/lectures#11.