OSS: install node via nvm on iOS image (#32309)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/32309

With the iOS image using Xcode 13, the Circle CI guide mentioned to use `nvm` to manage node: https://circleci.com/docs/2.0/testing-ios/#images-using-xcode-13-and-later.

Note: doing this here also helps with Xcode build scripts that needs to invoke this line: https://github.com/facebook/react-native/blob/main/scripts/find-node.sh#L27. For some reason, with Xcode 13 Circle CI image, without explicitly sourcing find-node.sh beforehand, react-native-codegen script phase within Xcode would fail (permission denied), like: https://app.circleci.com/pipelines/github/facebook/react-native/10525/workflows/b68b10a3-325a-4892-8252-baed9017c613/jobs/219615

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D31331905

fbshipit-source-id: 387c7a8931b07ee76fa6cd081d04c8704c1f7da4

.circleci/config.yml

@@ -370,14 +370,18 @@ jobs:
           steps:
             - brew_install:
                 package: watchman
-            - brew_install:
-                package: node@12
             - run:
                 name: "Brew: Tap wix/brew"
                 command: HOMEBREW_NO_AUTO_UPDATE=1 brew tap wix/brew >/dev/null
             - brew_install:
                 package: applesimutils
 
+      - run:
+          name: Configure Node
+          # Sourcing find-node.sh will ensure nvm is set up.
+          # It also helps future invocation of find-node.sh prevent permission issue with nvm.sh.
+          command: source scripts/find-node.sh && nvm install 12 && nvm alias default 12
+
       - run:
           name: Configure Watchman
           command: echo "{}" > .watchmanconfig