Set NODE_ENV=production during installation

This prevents installation of devDependencies in the current project.
There are none at the moment for both nextstrain.org and auspice.us, but
if that ever changes in the future, we would want to ignore them in this
step.

.github/workflows/preview_on_downstream_repo.yaml

@@ -41,6 +41,8 @@ jobs:
       - name: Install Auspice from PRs HEAD commit
         shell: bash
         working-directory: ${{ inputs.directory }}
+        env:
+          NODE_ENV: production
         run: |
           npm config set ignore-scripts true
           npm ci