feat: add save-cache option for PR-scoped build caching

[mmcky]

Add optional save-cache input to restore-jupyter-cache for PR-scoped build caching.

Problem

When a PR author pushes a fix and the CI re-runs, the entire build re-executes from scratch (using only the weekly main cache as a starting point). This means every push to a PR triggers a full 10-20 minute build.

Solution

Add a save-cache: 'true' option that switches from actions/cache/restore (read-only) to actions/cache (restore + auto-save at job end). The cache key uses github.run_id for uniqueness, and restore-keys prefix matching finds the most recent prior cache.

How it works

1. First PR run: Restores weekly cache from main → full build → saves result as PR-scoped cache
2. Second+ PR runs: Restores the PR's own cache → only re-executes changed notebooks → saves updated cache

Cache isolation (GitHub Actions scoping)

• Caches created on main → readable by all branches/PRs
• Caches created on a PR branch → only readable by that same PR
• No risk of PR caches affecting other PRs or main

Usage

- uses: quantecon/actions/restore-jupyter-cache@v0
  with:
    cache-type: 'build'
    source-dir: 'lectures'
    save-cache: 'true'  # Save build cache for faster subsequent PR runs

Backward compatible

Default is save-cache: 'false' — existing workflows are unaffected.