GH Action: Only publish to NPM when tagged (#3453)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/3453

Per title. Otherwise leads to continuous errors on every
commit like these: https://github.com/facebook/flipper/runs/5188689903?check_suite_focus=true

Reviewed By: nikoant

Differential Revision: D34216898

fbshipit-source-id: 686abdca5e70bee62917bcb8db265dcea5fcbb98

.github/workflows/release.yml

@@ -216,6 +216,7 @@ jobs:
     - name: Set up npm token
       run: echo "//registry.yarnpkg.com/:_authToken=${{ secrets.FLIPPER_NPM_TOKEN }}" >> ~/.npmrc
     - name: Publish flipper-server on NPM
+      if: ${{ needs.release.outputs.tag != '' }}
       run: |
         tar zxvf flipper-server.tgz/flipper-server.tgz
         cd package