nodejs version compatible with gitbook-cli

Explicitly specify latest version of nodejs which is compatible with abandoned 'gitbook-cli' v2.3.2

Author: gavvvr

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
  - uses: actions/checkout@v1
  - uses: actions/setup-node@v1
  with:
- node-version: 12
+ node-version: '12.18.2'
  - run: sudo apt update
  - run: sudo apt install calibre-bin
  - run: sudo npm install -g gitbook-cli