chore: hardcode node 20 in pr-preview workflow

Aligns with the node version used in the existing CI workflow
rather than reading from .nvmrc which specifies v18.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nicolethoen

.github/workflows/pr-preview.yml

@@ -57,7 +57,7 @@ jobs:
           git checkout tmp
       - uses: actions/setup-node@v4
         with:
-          node-version-file: '.nvmrc'
+          node-version: '20'
       - run: corepack enable
       - uses: actions/cache@v4
         id: yarn-cache