restore 3.1.x for publish script

.github/workflows/publish.yml

@@ -12,11 +12,17 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
-    - name: Setup .NET Core
+
+    - name: Setup .NET 5
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: 5.0.*
 
+    - name: Setup .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.*
+
     - name: Build with dotnet
       run: ./build.sh --linksources=true --verbosity=verbose
       shell: bash