Explicitly set framework for dotnet invocations

It seems `TargetFrameworks` is conflicting with our `global.json` CI step; we
are locking the SDK to a specific version but the csproj sometimes specifies a
higher target framework.

.github/workflows/dotnet.yml

@@ -26,9 +26,11 @@ jobs:
     # By default, tests will be executed under the latest installed version!
     - name: Create temporary global.json
       run: echo '{"sdk":{"version":"${{steps.stepid.outputs.dotnet-version}}"}}' > ./global.json
-    - name: Restore nuget dependencies
-      run: dotnet restore
+    # - name: Restore nuget dependencies
+    #   run: dotnet restore
+    # - name: Build solution
+    #   run: dotnet build --no-restore
     - name: Build solution
-      run: dotnet build --no-restore
+      run: dotnet build --framework ${{ matrix.dotnet.framework }}
     - name: Run tests
-      run: dotnet test --no-build --verbosity normal
+      run: dotnet test --no-build --verbosity normal --framework ${{ matrix.dotnet.framework }}