Add a workflow step to do dotnet restore

Author: edoardopirovano

.github/workflows/pr-checks.yml

@@ -370,6 +370,9 @@ jobs:
         run: |
           ../action/runner/dist/codeql-runner-macos init --repository $GITHUB_REPOSITORY --languages csharp --github-url $GITHUB_SERVER_URL --github-auth ${{ github.token }}
 
+      - name: Initialize dotnet
+        run: dotnet restore
+
       - name: Build code
         shell: bash
         run: |