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-gpu/test_gpu_demos.py

@@ -1,10 +1,13 @@
 import os
 import subprocess
 import sys
+import pytest
 sys.path.append("tests/python")
+import testing as tm
 import test_demos as td         # noqa
 
 
+@pytest.mark.skipif(**tm.no_cupy())
 def test_data_iterator():
     script = os.path.join(td.PYTHON_DEMO_DIR, 'data_iterator.py')
     cmd = ['python', script]