Fix caching errors during Firestore build (#5763)

Author: schmidt-sebastian

packages/firestore/rollup.config.js

@@ -84,7 +84,6 @@ const browserPlugins = function () {
         }
       },
       cacheDir: tmp.dirSync(),
-      clean: true,
       abortOnError: false,
       transformers: [util.removeAssertAndPrefixInternalTransformer]
     }),

packages/firestore/test/unit/specs/describe_spec.ts

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import * as stringify from 'json-stable-stringify';
+import stringify from 'json-stable-stringify';
 import { ExclusiveTestFunction, PendingTestFunction } from 'mocha';
 
 import { queryEquals, QueryImpl } from '../../../src/core/query';