fix(@angular-devkit/build-angular): pin copy-webpack-plugin

Fix angular#12553
Fix angular#12260

Author: filipesilva

packages/angular_devkit/build_angular/package.json

@@ -15,7 +15,7 @@
     "autoprefixer": "^8.4.1",
     "circular-dependency-plugin": "^5.0.2",
     "clean-css": "^4.1.11",
-    "copy-webpack-plugin": "^4.5.2",
+    "copy-webpack-plugin": "4.5.2",
     "file-loader": "^1.1.11",
     "glob": "^7.0.3",
     "html-webpack-plugin": "^3.0.6",