Update Create Pull Request GitHub workflow reviewers (#161005)

Update reviewer.

I didn't know this existed until I saw
https://github.com/flutter/flutter/pull/161001.
Introduced https://github.com/flutter/flutter/pull/140363. Tested
credentials are used in the cherry-pick bot:

3762f2e973/.github/workflows/easy-cp.yml (L69)

.github/workflows/minimal.yml

@@ -39,4 +39,6 @@ jobs:
           labels: |
             cp: review
           title: '[github actions] testing purposes'
-          reviewers: xilaizhang
+          reviewers:
+            - christopherfujino
+            - jmagman