ci: fix ERR_PNPM_ADDING_TO_ROOT in create-payload-app int (#14581)

`ERR_PNPM_ADDING_TO_ROOT` errors randomly started in the
create-payload-app int suite despite no changes in related code on our
end.

Solving this by ignoring workspace root with pnpm. It is still unclear
what in this test flow actually changed in CI.

---
Investigation:
- Last successful runs: ~17 hours ago (2025-11-11 ~20:51 UTC)
- Failures started: ~12 hours ago (2025-11-11 ~11:00 UTC)
- 100% of integration tests fail, e2e tests pass

1. Code Changes: NONE

- No PRs merged to main between last success and first failure
- No changes to:
  - create-payload-app package
  - Workspace config (.npmrc, pnpm-workspace.yaml)
  - pnpm version (still 9.7.1 in .tool-versions)
  - Test files

2. GitHub Actions Environment

- Runner image: ubuntu-24.04 (20251102.99.1) - deployed Nov 3, 2025
- No breaking changes found in Nov image update
- pnpm/action-setup@v4: Latest is v4.2.0 (Oct 2024) - no recent updates

3. pnpm Behavior 
- ✓ pnpm 9.7.1 released Aug 2024 (no recent changes)
- No changes to ERR_PNPM_ADDING_TO_ROOT logic in 9.7.1
- Error occurs when pnpm detects directory as workspace root

6. Test Flow

- Test creates temp directory
- scaffolds Next.js with pnpm create next-app@canary
- calls pnpm add payload@latest...
- pnpm rejects, treating temp directory as workspace root

Author: denolfe

packages/create-payload-app/src/lib/install-packages.ts

@@ -23,6 +23,10 @@ export async function installPackages(args: {
       }
       ;({ exitCode, stderr } = await execa(packageManager, ['add', ...packagesToInstall], {
         cwd: projectDir,
+        env: {
+          ...process.env,
+          ...(packageManager === 'pnpm' && { npm_config_ignore_workspace_root_check: 'true' }),
+        },
       }))
       break
     }