[Azure] Limit ninja build to two jobs. [ci skip]

https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops

-> Standard_DS2_v2: 2 cores, 7 GB Ram

azure-pipelines.yml

@@ -21,5 +21,5 @@ jobs:
       cd build
       call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\Tools\vsdevcmd" -arch=x64
       cmake .. -G Ninja -DCMAKE_C_COMPILER=cl.exe -DCMAKE_CXX_COMPILER=cl.exe -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF -DOGS_EIGEN_DYNAMIC_SHAPE_MATRICES=ON -DOGS_USE_PCH=OFF
-      ninja tests
-      ninja ctest
+      ninja -j 2 tests
+      ninja -j 2 ctest