ci: ensure both net5 & net6 are installed

.vsts-ci.yml

@@ -46,6 +46,12 @@ jobs:
       configFilePath: gitversion.yml
     displayName: Use GitVersion
 
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core SDK 5.0.101'
+    inputs:
+      packageType: sdk
+      version: 5.0.101
+
   - task: UseDotNet@2
     displayName: 'Use .NET Core SDK 6.0.401'
     retryCountOnTaskFailure: 3
@@ -104,6 +110,13 @@ jobs:
     clean: true
 
   - task: UseDotNet@2
+    displayName: 'Use .NET Core SDK 5.0.101'
+    inputs:
+      packageType: sdk
+      version: 5.0.101
+
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core SDK 6.0.401'
     inputs:
       packageType: sdk
       version: 6.0.401