ci: put the npm step back in for only Windows

Author: phated

.ci/.azure-pipelines-steps.yml

@@ -1,12 +1,13 @@
 steps:
+- script: npm i -g npm@$(npm_version)
+  displayName: Use non-broken npm version $(npm_version)
+  condition: ne(variables['npm_version'], '')
+
 - task: NodeTool@0
   inputs:
     versionSpec: '$(node_version)'
   displayName: Use Node $(node_version)
 
-- script: npm install -g npm@6.4.1
-  displayName: Revert npm to 6.4.1 (known working)
-
 - script: npm install
   displayName: npm install
 

.ci/.azure-pipelines.yml

@@ -26,8 +26,10 @@ jobs:
       matrix:
         Node_v10:
           node_version: 10
+          npm_version: 6.4.1
         Node_v8:
           node_version: 8
+          npm_version: 6.4.1
         Node_v6:
           node_version: 6
     steps: