add yarn install step

Author: abillingsley

.github/workflows/npmpublish.yml

@@ -16,6 +16,7 @@ jobs:
         with:
           node-version: 12
           registry-url: "https://registry.npmjs.org"
+      - run: yarn install --frozen-lockfile
       - run: npm publish --access public
         env:
           NODE_AUTH_TOKEN: ${{secrets.NPM_PUBLISHING_TOKEN}}