test: Update npm to latest in test_node_templates workflow (#611)

Ensures the `test_node_templates.yaml` workflow uses the latest npm
version instead of the bundled version that ships with each Node.js
release.

## Changes

- Added `npm install -g npm@latest` step after Node.js setup and before
dependency installation
- Step executes across all matrix combinations (Node 20, 22, 24 on
Ubuntu and Windows)

This updates npm from 10.8.2 (bundled) to 11.7.0+ (latest) for
consistent testing with current npm features and fixes.

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<details>

<summary>Original prompt</summary>

> update npm to latest in this flow:
https://github.com/apify/actor-templates/blob/master/.github/workflows/test_node_templates.yaml


</details>



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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: B4nan <[email protected]>

Author: Copilot

.github/workflows/test_node_templates.yaml

@@ -24,6 +24,9 @@ jobs:
           node-version: ${{ matrix.node-version }}
           cache: 'npm'
 
+      - name: Update npm to latest
+        run: npm install -g npm@latest
+
       - name: Cache node_modules
         uses: actions/cache@v4
         with: