fix: pnp test cases failed when using rspack (#79084)

The Yarn PnP test suite (`test/e2e/yarn-pnp/test/*.test.ts`) fails when
running with Rspack because tests don't install required `@rspack/core`
dependency.

test/e2e/yarn-pnp/test/utils.ts

@@ -35,6 +35,10 @@ export function runTests(
       delete packageJson.dependencies['react']
       delete packageJson.dependencies['react-dom']
 
+      if (process.env.NEXT_RSPACK) {
+        packageJson.dependencies['@rspack/core'] = 'latest'
+      }
+
       next = await createNext({
         files: srcFiles.reduce(
           (prev, file) => {