Switch CodeSandbox repros to pnpm

[karlhorky]

Closes #57895
Closes #57896

What?

Switch Yarn v1 to pnpm for reproduction repos, avoiding the CodeSandbox Yarn v1 yarn.lock caching problem described below

Why?

As confirmed with @CompuIves, CodeSandbox caches the yarn.lock file generated after the first start + yarn install of a GitHub template (Yarn v1) - this saves old versions of Next.js (not the latest canary) in the lockfile and causes old versions of Next.js to be installed when users click on the CodeSandbox Reproduction link during issue creation.

This leads to the current breakage, where Next.js actually cannot even start in the reproduction sandbox:

1. Start creating a new GitHub issue on the vercel/next GitHub repo
2. Open the CodeSandbox reproduction link linked in the Link to the code that reproduces this issue section
3. Observe the error message below in the dev task started below (experimental.appDir: true was required in the 13.0.8-canary.0 version of Next.js in the cached yarn.lock file, but the next.config.js file no longer includes this config)

$ yarn dev
yarn run v1.22.19
$ next dev
ready - started server on 0.0.0.0:3000, url: http://localhost:3000
Error: > The `app` directory is experimental. To enable, add `appDir: true` to your `next.config.js` configuration under `experimental`. See https://nextjs.org/docs/messages/experimental-app-dir-config
    at Object.findPagesDir (/project/sandbox/node_modules/next/dist/lib/find-pages-dir.js:80:19)
    at new DevServer (/project/sandbox/node_modules/next/dist/server/dev/next-dev-server.js:110:59)
    at NextServer.createServer (/project/sandbox/node_modules/next/dist/server/next.js:140:20)
    at /project/sandbox/node_modules/next/dist/server/next.js:155:42
    at async NextServer.prepare (/project/sandbox/node_modules/next/dist/server/next.js:130:24)
    at async /project/sandbox/node_modules/next/dist/cli/next-dev.js:344:17
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Sandbox demo: https://codesandbox.io/p/sandbox/quirky-pascal-n32xk2?file=%2Fnext.config.js%3A5%2C1

How?

Switch to pnpm installation commands instead of Yarn v1

Caveats

This uses CodeSandbox-specific config

Alternatives considered

• Delete the outdated yarn.lock file on setup of the sandbox, PR open here: Remove outdated yarn.lock in CodeSandbox repros #57895
• Commit an empty yarn.lock file to block caching of this file, PR open here: Add empty yarn.lock to prevent caching #57896

cc @samcx @CompuIves

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 835bfe2

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: d50e77c

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[karlhorky]

@styfle Thanks for the review and merge!

Just confirmed - sandboxes created from the CodeSandbox reproduction link are working again 🚀