Pinned Node version in CI

- The default Node version on GitHub Actions is moving to Node 14 and we
  want to keep building it on 12 for now, so this commit pins the
  version to 12.18.0

.github/workflows/build.yml

@@ -12,6 +12,9 @@ jobs:
         with:
           fetch-depth: 0
           submodules: true
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '12.18.0'
 
       - name: Set up Git
         run: |

.github/workflows/release.yml

@@ -15,6 +15,10 @@ jobs:
         with:
           fetch-depth: 0
           submodules: true
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '12.18.0'
+
       - run: yarn
       - run: grunt release --skip-tests
       - run: npm config set //registry.npmjs.org/:_authToken=$NODE_AUTH_TOKEN