Use Node.js version 20

This change doesn't really affect the installation speed, but it's good
to be consistent with the version used to build both nextstrain.org and
auspice.us.

.github/workflows/preview_on_downstream_repo.yaml

@@ -30,7 +30,7 @@ jobs:
     steps:
       - uses: actions/setup-node@v4
         with:
-          node-version: '16'
+          node-version: '20'
 
       - name: Checkout downstream repo
         uses: actions/checkout@v4