Fix windows multisdk

Author: macsux

.github/workflows/dotnet.yaml

@@ -26,6 +26,13 @@ jobs:
       - name: .net SxS
         run: |
           rsync -a ${DOTNET_ROOT/3.1.200/2.1.300}/* $DOTNET_ROOT/
+
+      - name: Setup side by side .NET SDKs on Windows
+        if: matrix.os == 'windows-latest'
+        run: |
+          SET DOTNET_21_ROOT=%DOTNET_ROOT:3.1.200=2.1.300%
+          xcopy /s /y /d %DOTNET_21_ROOT% %DOTNET_ROOT%
+        shell: cmd
       - run: dotnet build --configuration Release
       - name: test with dotnet
         run: dotnet test /p:CollectCoverage=true /p:ExcludeByFile=\"**/KubernetesClient/generated/**/*.cs\" /p:CoverletOutputFormat="cobertura"