refactor: Add explict type to allow RxJS to be updated

While trying to sync RxJS into google3 an issue came up around the code in question, where TypeScript is unable to properly infer the type. Adding this explicit type resolves the issue.

Author: benlesh

packages/angular_devkit/core/src/experimental/workspace/workspace.ts

@@ -17,6 +17,7 @@ import {
   parseJson,
   schema,
 } from '../../json';
+import { SchemaValidatorResult } from '../../json/schema/interface';
 import {
   Path,
   basename,
@@ -342,7 +343,7 @@ export class Workspace {
 
     return this._registry.compile(schemaJson).pipe(
       concatMap(validator => validator(contentJsonCopy)),
-      concatMap(validatorResult => {
+      concatMap((validatorResult: SchemaValidatorResult) => {
         if (validatorResult.success) {
           return of(contentJsonCopy as T);
         } else {