test: create node_modules directory in PnP integration test

The Angular CLI requires the `node_modules` directory for ngcc to be
present. With PnP this directory does not exist and we need to make sure
it exists. Previously it worked without the explicit command because it
went up the execroot, but the CLI made this stricter so that we now
notice it until the fix/looser check is available:

angular/angular-cli@5319428.

Author: devversion

integration/yarn-pnp-compat/BUILD.bazel

@@ -16,6 +16,9 @@ node_integration_test(
     srcs = glob(["**/*"]),
     commands = [
         "yarn install --no-immutable",
+        # TODO(devversion): Remove when https://github.com/angular/angular-cli/commit/5319428e14a7e364a58caa8ca936964cfc5503cf
+        # is available.
+        "mkdir node_modules",
         "yarn build",
     ],
     environment = {