fix(@angular-devkit/build-angular): pin clean-css

We need clean-css/clean-css#1050 to be fixed before updating to 4.2.x.

Fix angular#12408

Author: filipesilva

packages/angular_devkit/build_angular/package.json

@@ -14,7 +14,7 @@
     "ajv": "6.5.3",
     "autoprefixer": "8.4.1",
     "circular-dependency-plugin": "5.0.2",
-    "clean-css": "4.2.1",
+    "clean-css": "~4.1.11",
     "copy-webpack-plugin": "4.5.2",
     "file-loader": "2.0.0",
     "glob": "7.1.3",