feat(@angular/cli): add support for custom npmrc paths

According to [the npm config
docs](https://docs.npmjs.com/misc/config#npmrc-files), a user can
specify the path to their `npmrc` files using the environment
variables `NPM_CONFIG_USERCONFIG` and `NPM_CONFIG_GLOBALCONFIG`. When
set, these variables should override the default paths.

packages/angular/cli/utilities/package-metadata.ts

@@ -91,8 +91,8 @@ function readOptions(
   }
 
   const defaultConfigLocations = [
-    path.join(globalPrefix, 'etc', baseFilename),
-    path.join(homedir(), dotFilename),
+    (!yarn && process.env.NPM_CONFIG_GLOBALCONFIG) || path.join(globalPrefix, 'etc', baseFilename),
+    (!yarn && process.env.NPM_CONFIG_USERCONFIG) || path.join(homedir(), dotFilename),
   ];
 
   const projectConfigLocations: string[] = [