fix: ensure that the right version of NodeJS is installed

It's smart to keep the NodeJS version pinned so we can run all build jobs
consistently.

Otherwise, we might run into some versions mismatch like below:
```
npm ERR! While resolving: @typescript-eslint/parser@2.30.0
npm ERR! Found: eslint@7.4.0
npm ERR! node_modules/eslint
npm ERR!   dev eslint@"^7.4.0" from the root project
npm ERR!   peer eslint@"^5.0.0 || ^6.0.0 || ^7.0.0" from @typescript-eslint/eslint-plugin@4.11.1
npm ERR!   node_modules/@typescript-eslint/eslint-plugin
npm ERR!     @typescript-eslint/eslint-plugin@">=2.25.0" from eslint-plugin-github@4.1.1
npm ERR!     node_modules/eslint-plugin-github
npm ERR!       dev eslint-plugin-github@"^4.1.1" from the root project
npm ERR!   7 more (@typescript-eslint/experimental-utils, ...)
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer eslint@"^5.0.0 || ^6.0.0" from @typescript-eslint/parser@2.30.0
npm ERR! node_modules/@typescript-eslint/parser
npm ERR!   dev @typescript-eslint/parser@"^2.30.0" from the root project
npm ERR!   @typescript-eslint/parser@">=2.25.0" from eslint-plugin-github@4.1.1
npm ERR!   node_modules/eslint-plugin-github
npm ERR!     dev eslint-plugin-github@"^4.1.1" from the root project
npm ERR!
npm ERR! Conflicting peer dependency: eslint@6.8.0
npm ERR! node_modules/eslint
npm ERR!   peer eslint@"^5.0.0 || ^6.0.0" from @typescript-eslint/parser@2.30.0
npm ERR!   node_modules/@typescript-eslint/parser
npm ERR!     dev @typescript-eslint/parser@"^2.30.0" from the root project
npm ERR!     @typescript-eslint/parser@">=2.25.0" from eslint-plugin-github@4.1.1
npm ERR!     node_modules/eslint-plugin-github
npm ERR!       dev eslint-plugin-github@"^4.1.1" from the root project
```

.github/workflows/licensed.yml

@@ -14,6 +14,9 @@ jobs:
     name: Check licenses
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
       - run: npm ci
       - name: Install licensed
         run: |