fix: bump node version to 18 in build-addons CI job (#5965)

@aws-sdk/client-s3 v3 requires Node 18+; the ??= operator used by
@smithy/util-retry causes a SyntaxError on Node 14.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: emosbaugh

.github/workflows/deploy-staging.yaml

@@ -130,7 +130,7 @@ jobs:
     - uses: actions/checkout@v6
     - uses: actions/setup-node@v6
       with:
-        node-version: '14'
+        node-version: '18'
     - run: npm install
     - run: make generate-addons
       env: