ci: scope node_modules cache key per matrix node version

Address doistbot review.

Author: scottlovegrove

.github/workflows/pull_request.yml

@@ -26,7 +26,7 @@ jobs:
               id: node-modules-cache
               uses: actions/cache@v4
               with:
-                  key: node-modules-cache-${{ hashFiles('**/package-lock.json', '**/.node-version') }}
+                  key: node-modules-cache-${{ matrix.node-version }}-${{ hashFiles('**/package-lock.json', '**/.node-version') }}
                   path: node_modules/
 
             - name: Install project npm dependencies