Clarify e2e dependency on yarn in contributin docs (#43287)

`yarn` installed with `corepack` won't run in our repo because it is configured as a `pnpm` project. `yarn` binaries installed from other sources don't care.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

.github/actions/next-stats-action/src/prepare/repo-setup.js

@@ -131,7 +131,13 @@ module.exports = (actionInfo) => {
       // to the correct versions
       for (const pkgName of pkgDatas.keys()) {
         const { pkg, pkgPath } = pkgDatas.get(pkgName)
-        await exec(`cd ${pkgPath} && yarn pack -f ${pkg}-packed.tgz`, true)
+        await exec(`cd ${pkgPath} && yarn pack -f ${pkg}-packed.tgz`, true, {
+          env: {
+            // Yarn installed through corepack will not run in pnpm project without this env var set
+            // This var works for corepack >=0.15.0
+            COREPACK_ENABLE_STRICT: '0',
+          },
+        })
       }
       return pkgPaths
     },

contributing/core/testing.md

@@ -33,6 +33,13 @@ When the test runs it will open the browser that is in the background by default
 pnpm testonly test/integration/production/ -t "should allow etag header support"
 ```
 
+**End-to-end (e2e)** tests are run in complete isolation from the repository.
+When you run an e2e test, a local version of next will be created inside your system's temp folder (eg. /tmp),
+which is then linked to the app, also created inside a temp folder. A server is started on a random port, against which the tests will run.
+After all tests have finished, the server is destroyed and all remaining files are deleted from the temp folder.
+
+You will need `yarn` for running e2e tests. Installing it `corepack` won't work because `next.js` is `pnpm` workspace.
+
 ## Writing tests for Next.js
 
 ### Getting Started