Use Node v14 in Windows CircleCI jobs (facebook#31656)

Summary:
Pull Request resolved: facebook#31656

CircleCI's Windows executor currently ships with a pre-LTS release of Node v12, breaking our Windows jobs ([example](https://app.circleci.com/pipelines/github/facebook/react-native/9280/workflows/21e6e59c-d853-47a1-af62-1368c8ce10ce/jobs/203983)) following facebook#30637, ultimately due to jestjs/jest#10685 dropping support for non-LTS versions in the Node v12 release line.

Luckily, the Windows executor [does ship with nvm](circleci/circleci-docs#3733) so we can use that to install a desired Node version. Rather than just pinning a later v12 release that is LTS, we align the Windows config with the one for non-Windows jobs (in the `nodelts` executor) by running against the latest available Node v14 release.

Changelog:
[Internal]

Differential Revision: D28896581

fbshipit-source-id: ed490bcdae80e736338c387e255191d8be1888cb

.circleci/config.yml

@@ -27,6 +27,7 @@ executors:
   nodelts:
     <<: *defaults
     docker:
+      # Note: Version set separately for Windows builds, see below.
       - image: circleci/node:14
   nodeprevlts:
     <<: *defaults
@@ -624,6 +625,11 @@ jobs:
     steps:
       - checkout
 
+      - run:
+          name: Install Node
+          # Note: Version set separately for non-Windows builds, see above.
+          command: nvm install 14 && nvm use 14
+
       # Setup Dependencies
       - run:
           name: Install Yarn