test(@angular-devkit/build-angular): pin node-sass to version 4

The curent version of sass-loader doesn't support node-sass version 5.

webpack-contrib/sass-loader#898
(cherry picked from commit 6717d18)

tests/legacy-cli/e2e/tests/build/styles/node-sass.ts

@@ -32,7 +32,7 @@ export default async function () {
   await silentExec('rm', '-rf', 'node_modules/sass');
   await expectToFail(() => ng('build', '--extract-css', '--source-map'));
 
-  await silentNpm('install', 'node-sass');
+  await silentNpm('install', 'node-sass@4');
   await silentExec('rm', '-rf', 'node_modules/sass');
   await ng('build', '--extract-css', '--source-map');