DMG build: Add missing yarn install

Summary: This happened: https://github.com/facebook/flipper/runs/496468216?check_suite_focus=true

Reviewed By: mweststrate

Differential Revision: D20361438

fbshipit-source-id: 5b9649ccabf8cc06fbdf28e5f7b45bd7fb5204a9

.github/workflows/dmg-release.yml

@@ -14,6 +14,8 @@ jobs:
     - uses: actions/setup-node@v1
       with:
         node-version: '12.x'
+    - name: Install
+      run: yarn
     - name: Build
       run: yarn build --mac --mac-dmg
     - name: Upload