Replace @npm_bazel_rollup with @npm//@bazel/rollup

This is needed after we have rules_nodejs upgraded to 2.0

Change-Id: I2c49a8a4d8bd0fe225c5218e8683e0f0aaffd7de

BUILD

diff --git a/BUILD b/BUILD
index 543849a..023a32b 100644
--- a/BUILD
+++ b/BUILD
@@ -1,6 +1,6 @@
 load("//tools/bzl:plugin.bzl", "gerrit_plugin")
 load("//tools/bzl:js.bzl", "polygerrit_plugin")
-load("@npm_bazel_rollup//:index.bzl", "rollup_bundle")
+load("@npm//@bazel/rollup:index.bzl", "rollup_bundle")
 
 gerrit_plugin(
     name = "chromium_coverage",