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

 * Not installing via `apt-get install dotnet-sdk-10.0` because that
   would lead to older runtimes being uninstalled.

 * Installing to `/usr/local/dotnet` SDK as it would not get detected
   in `~/.dotnet`.

.NET 10 SDK won't get detected if installed in `~/.dotnet`

Author: stakx

appveyor.yml

@@ -18,6 +18,12 @@ for:
     only:
     - image: Ubuntu2204
 
+  # install .NET 10 SDK
+  install:
+  - curl -L https://dot.net/v1/dotnet-install.sh -o dotnet-install.sh
+  - chmod +x ./dotnet-install.sh
+  - sudo ./dotnet-install.sh --channel 10.0 --install-dir /usr/share/dotnet
+
   # build and run tests
   build_script:
   - uname -a
@@ -46,6 +52,10 @@ 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