fix(@angular-devkit/build-angular): ensure NG_PERSISTENT_BUILD_CACHE always creates a cache in the specified cache directory

This change fixes `NG_PERSISTENT_BUILD_CACHE` sometimes creating cache
entries that live outside of the cache directory by using a hex encoding
rather than a base64 encoding. This error is caused because the base64
alphabet includes `/`. According to the webpack documentation [1] the
default `cacheLocation` is:

  path.resolve(cache.cacheDirectory, cache.name)

Which means cache names with a leading `/` would remove the
`cacheDirectory` altogether.

[1]: https://webpack.js.org/configuration/other-options/#cachecachelocation

Author: terencehonles

packages/angular_devkit/build_angular/src/webpack/configs/common.ts

@@ -532,7 +532,7 @@ function getCacheSettings(
         .update(JSON.stringify(wco.tsConfig))
         .update(JSON.stringify(wco.buildOptions))
         .update(supportedBrowsers.join(''))
-        .digest('base64'),
+        .digest('hex'),
     };
   }