feat(ci): add Lake cache shadow workflow for master (leanprover-community#39402)

Introduces a scheduled, non-blocking pipeline that exercises Lake's built-in artifact cache against mathlib master, tracking the eventual migration off `lake exe cache`

The workflow runs daily on cron and on workflow_dispatch. Jobs:

build_and_stage:
1. Hydrates .lake/build/ via legacy `lake exe cache get`
2. patches the lakefile to enable Lake's artifact cache with three knobs
3. runs an incremental `lake build` to populate Lake's cache from existing artifacts
4. uploads cache artifact for next job.

upload: Downloads the staging artifact and pushes it to the cache bucket via `lake cache put-staged`.

consume: 
- `lake cache get` from the cache bucket
- `lake build` against the rehydrated cache, `--rehash` verify.

Author: marcelolynch