fix node-version to a specific version in ci.yml

to avoid issues downloading new versions
see actions/setup-node#160 (comment) for more context

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 19.x
+          node-version: 18.18
           cache: "npm"
       - run: npm ci
       - run: npm run prettier:check
@@ -24,7 +24,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: 19.x
+          node-version: 18.18
           cache: "npm"
       - run: npm ci
       - run: npm test