Make internal-only steps advanced.

This is so they don't show up in the Snippet Generator or the eventual
Pipeline Editor step choice UI, since they only exist for internal purposes.

pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/steps/DockerLabelStep.java

@@ -57,6 +57,11 @@ public DescriptorImpl() {
             super(StepExecutionImpl.class);
         }
 
+        @Override
+        public boolean isAdvanced() {
+            return true;
+        }
+
         @Override
         public String getFunctionName() {
             return "dockerLabel";

pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/steps/EnvVarsForToolStep.java

@@ -82,6 +82,11 @@ public DescriptorImpl() {
             super(EnvVarsForToolStepExecution.class);
         }
 
+        @Override
+        public boolean isAdvanced() {
+            return true;
+        }
+
         @Override public String getFunctionName() {
             return "envVarsForTool";
         }