Fix build

Author: RehanSaeed

.github/workflows/build.yml

@@ -37,6 +37,7 @@ jobs:
         uses: actions/[email protected]
         with:
           dotnet-version: |
+            3.1.x
             6.0.x
           global-json-file: "./global.json"
       - name: "Dotnet Tool Restore"