add .NET 3.1 SDK to CI machines (#10986) (#10988)

Co-authored-by: Brett V. Forsgren <[email protected]>

Author: KevinRansom

azure-pipelines.yml

@@ -56,6 +56,8 @@ variables:
     value: .NETCore
   - name: VisualStudioDropName
     value: Products/$(System.TeamProject)/$(Build.Repository.Name)/$(Build.SourceBranchName)/$(Build.BuildNumber)
+  - name: LegacyDotNetSdkVersion
+    value: 3.1.405
   - name: DotNetSdkVersion
     value: '5.0.100'
   - ${{ if and(eq(variables['System.TeamProject'], 'public'), eq(variables['Build.Reason'], 'PullRequest')) }}:
@@ -103,6 +105,12 @@ stages:
           steps:
           - checkout: self
             clean: true
+          - task: UseDotNet@2
+            displayName: Add legacy .NET SDK
+            inputs:
+              packageType: sdk
+              version: $(LegacyDotNetSdkVersion)
+              installationPath: $(Agent.ToolsDirectory)/dotnet
           - script: eng\CIBuild.cmd
                     -configuration $(_BuildConfig)
                     -prepareMachine
@@ -211,6 +219,12 @@ stages:
           steps:
           - checkout: self
             clean: true
+          - task: UseDotNet@2
+            displayName: Add legacy .NET SDK
+            inputs:
+              packageType: sdk
+              version: $(LegacyDotNetSdkVersion)
+              installationPath: $(Agent.ToolsDirectory)/dotnet
           - script: eng\CIBuild.cmd -configuration $(_configuration) -$(_testKind)
             displayName: Build / Test
           - task: PublishTestResults@2
@@ -261,6 +275,12 @@ stages:
           steps:
           - checkout: self
             clean: true
+          - task: UseDotNet@2
+            displayName: Add legacy .NET SDK
+            inputs:
+              packageType: sdk
+              version: $(LegacyDotNetSdkVersion)
+              installationPath: $(Agent.ToolsDirectory)/dotnet
           - script: ./eng/cibuild.sh --configuration $(_BuildConfig) --testcoreclr
             displayName: Build / Test
           - task: PublishTestResults@2
@@ -293,6 +313,12 @@ stages:
           steps:
           - checkout: self
             clean: true
+          - task: UseDotNet@2
+            displayName: Add legacy .NET SDK
+            inputs:
+              packageType: sdk
+              version: $(LegacyDotNetSdkVersion)
+              installationPath: $(Agent.ToolsDirectory)/dotnet
           - script: ./eng/cibuild.sh --configuration $(_BuildConfig) --testcoreclr
             displayName: Build / Test
           - task: PublishTestResults@2