AppVeyor: install .NET 10 SDK required for C# 14

Author: stakx

appveyor.yml

@@ -18,6 +18,10 @@ for:
     only:
     - image: Ubuntu2204
 
+  # install .NET 10 SDK
+  install:
+  - sudo apt-get update && sudo apt-get install -y dotnet-sdk-10.0
+
   # build and run tests
   build_script:
   - uname -a
@@ -46,6 +50,10 @@ for:
           Update-AppveyorBuild -Version ($env:APPVEYOR_REPO_TAG_NAME).TrimStart("v")
       }
 
+  # install .NET 10 SDK
+  install:
+  - cmd: choco install dotnet-10.0-sdk
+
   # build and run tests
   build_script:
   - cmd: build.cmd