Use the newest checkout and cache actions

[jakubfijalkowski]

Bumps the two third-party actions the repo uses. Both @v4 releases run on Node 20, which has reached end of life; the new majors run on Node 24.

• actions/checkout@v4 → @v7 in the example app preview workflow and the README example
• actions/cache@v4 → @v6 in action.yml, install, and install-sdk

None of the changes since v4 affect how these actions are used here:

• checkout v6 stores persisted credentials in a separate file, and v7 refuses to check out fork PR code on pull_request_target and workflow_run. The preview workflow runs on pull_request.
• cache v6 keeps the path, key, and restore-keys inputs and the cache-hit output, so sdk-cache-hit and cache-hit keep working.

Consumers running install, install-sdk, or the combined action on self-hosted runners now need runner 2.327.1 or newer for the Node 24 cache action. GitHub-hosted runners already meet this.

This PR touches action.yml, install*/**, and the workflow, so the example app preview run exercises both new versions.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SaRi7LTjpUQPvw8TKtvJZz

[github-actions]

📱 Flitz preview build

Open the landing page — scan its QR code, or open the link on the device.

Open it with a Flitz-enabled loader build.

Details

• Deeplink: flitz://download?url=https%3A%2F%2Fd.flitz.dev%2Forg_01KW9XB1HBQ6PCPH5QS7XK9HF8%2Fflitz_example-f81c81bc0b96.flitz
• Bundle: https://d.flitz.dev/org_01KW9XB1HBQ6PCPH5QS7XK9HF8/flitz_example-f81c81bc0b96.flitz
• Publish id: ae91c042-ab1c-43ae-8edb-85d88e54181a
• Commit: c2b3755a234e9d98bbf1d9943d526b98e731b3b0

Opening a bundle is transient — restart the app to return to the installed build.