Appveyor: Test only net8

Author: wallymathieu

appveyor.yml

@@ -18,7 +18,7 @@ install:
 build_script:
   - cmd: dotnet restore ./FSharpPlus.sln
   - cmd: dotnet build -c Release ./FSharpPlus.sln
-  - cmd: dotnet test -c Test tests/FSharpPlus.Tests
+  - cmd: dotnet test -f net8.0 -c Test tests/FSharpPlus.Tests
   - ps: if ($env:VersionSuffix) { dotnet pack build.proj --version-suffix $env:VersionSuffix } else { dotnet pack build.proj }
 test: off
 artifacts: