Adopt setup + post-step cache pattern (don't run commands in the action)

[javier-pm]

Problem

The action currently runs Lake commands (build, lint, mathlib cache) and saves the cache during its own execution, before any user-defined steps run. This means artifacts produced by subsequent steps (the user's own lake build, tests, etc.) are never cached.

Users who set build: false and run their own commands get no benefit from the cache — every CI run rebuilds from scratch.

Precedent: Gradle Actions

gradle/actions had the exact same problem. The original action ran Gradle commands and cached before user steps. They redesigned it into setup-gradle:

• Setup step: Install Gradle + restore cache
• Post step: Save cache (after all user steps complete)

This ensures the cache always reflects the final build state.

Proposal

The action's core responsibilities should be:

1. Setup step: Install the Lean toolchain + restore cache
2. Post step: Save cache after all user steps have run

Everything else — lake build, lake lint — should be the user's own workflow steps.

- uses: leanprover/lean-action@v2
  with:
    lake-package-directory: packages

# User controls what runs
- run: make build
- run: make lint
- run: make test

# Post step (automatic): lean-action saves the cache