ci: simplify backend unit test workflow by removing yarn installation and redundant setup steps

.github/workflows/backend-unit-test.yaml

@@ -10,18 +10,12 @@ jobs:
         uses: actions/setup-node@v4
         with:
           node-version: '20'
-      - name: Setup yarn # Needed for the redlock package
-        run: npm install -g yarn
       - name: Install wait-on
         run: npm install -g wait-on
       - name: Checkout OpenVidu Meet
         uses: actions/checkout@v4
       - name: Setup OpenVidu Meet
-        shell: bash
-        run: |
-          ./prepare.sh
-          cd backend
-          npm install
+        uses: ./.github/actions/setup-openvidu-meet
       - name: Run tests
         run: |
           cd backend