Further improvements and savings in Jenkins pipeline (#5904)

* Publish artifacts only on the master and release branches

* Build CUDA only for Compute Capability 7.5 when building PRs

* Run all Windows jobs in a single worker image

* Build nightly XGBoost4J SNAPSHOT JARs with Scala 2.12 only

* Show skipped Python tests on Windows

* Make Graphviz optional for Python tests

* Add back C++ tests

* Unstash xgboost_cpp_tests

* Fix label to CUDA 10.1

* Install cuPy for CUDA 10.1

* Install jsonschema

* Address reviewer's feedback

tests/python/test_with_sklearn.py

@@ -437,6 +437,7 @@ def test_sklearn_api_gblinear():
 
 
 @pytest.mark.skipif(**tm.no_matplotlib())
+@pytest.mark.skipif(**tm.no_graphviz())
 def test_sklearn_plotting():
     from sklearn.datasets import load_iris