[GitHub Actions] Use Xcode 12 on `pod lib lint`

.github/workflows/pod_lib_lint.yml

@@ -22,9 +22,9 @@ on:
 jobs:
   pod_lib_lint:
     name: pod lib lint
-    runs-on: macOS-10.14
+    runs-on: macOS-10.15
     env:
-      DEVELOPER_DIR: /Applications/Xcode_10.3.app
+      DEVELOPER_DIR: /Applications/Xcode_12.app
     steps:
       - uses: actions/checkout@v1
       - run: pod lib lint