use xcode 12.0

Author: liulianci

.github/workflows/ci.yml

@@ -24,6 +24,7 @@ jobs:
       - uses: actions/setup-node@v1
         with:
           node-version: '12'
+          xcode-version: '12.0'
       - run: npm i -g electrode-native
       - run: yarn --frozen-lockfile
       - working-directory: ios/