chore: upgrade PR build to use Node 14 (aws-samples#571)

The current PR build shows many warnings throughout due to still using Node 10,
which is EOL. This doesn't break anything, but is silly. Upgrade the workflow to
use Node 14.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/workflows/build-pull-request.yml

@@ -18,6 +18,11 @@ jobs:
         with:
           fetch-depth: 2
 
+      - name: Set up Node
+        uses: actions/[email protected]
+        with:
+          node-version: 14
+
       - name: Check files
         id: check_files
         run: |