try moving the 3.0 install to after the tool

azure-pipelines.rx.yml

@@ -116,10 +116,6 @@ jobs:
     inputs:
       version: '2.2.100'
 
-  - task: DotNetCoreInstaller@0
-    inputs:
-      version: '3.0.100-preview-009812'
-
   - task: DotNetCoreCLI@2  
     inputs:
       command: custom
@@ -130,6 +126,10 @@ jobs:
   - script: ./nbgv cloud -a -p Rx.NET/Source
     displayName: Set Version
 
+  - task: DotNetCoreInstaller@0
+    inputs:
+      version: '3.0.100-preview-009812'
+
   - task: DownloadBuildArtifacts@0
     displayName: 'Download Build Artifacts'
     inputs: