steps to pull node and yarn, then install them during circle container prep

gistfile1.yml

# step should have NODE_VERSION and YARN_VERSION environment variables declared 

      - run:
          name: Download NodeJS things
          command: |
            mkdir -p nodejs-downloads/
            cd nodejs-downloads/
            [ -f node-v${NODE_VERSION}-linux-x64.tar.xz ] || curl -O https://nodejs.org/dist/v${NODE_VERSION}/node-v${NODE_VERSION}-linux-x64.tar.xz
            [ -f yarn-v${YARN_VERSION}.tar.gz ] || curl -O -L https://yarnpkg.com/downloads/${YARN_VERSION}/yarn-v${YARN_VERSION}.tar.gz

      - run:
          name: Install NodeJS things
          command: |
            cd nodejs-downloads/
            sudo tar -xJ -C /usr/local --strip-components=1 --no-same-owner -f node-v${NODE_VERSION}-linux-x64.tar.xz
            sudo ln -s /usr/local/bin/node /usr/local/bin/nodejs
            sudo mkdir -p /opt/yarn
            sudo tar -xz -C /opt/yarn --strip-components=1 -f yarn-v${YARN_VERSION}.tar.gz
            sudo ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn
            sudo ln -s /opt/yarn/bin/yarn /usr/local/bin/yarnpkg