Add geckodriver to path

Author: jhamrick

azure-pipelines.yml

@@ -23,6 +23,10 @@ strategy:
       testgroup: 'nbextensions'
 
 steps:
+- script: |
+    echo ##vso[task.prependpath]$(GeckoWebDriver)
+  displayName: 'Add gecko driver to PATH'
+
 - script: |
     env | sort
   displayName: 'Show environment variables'