CI: Install .NET 8.0 manually on AppVeyor

appveyor.yml

@@ -11,6 +11,12 @@ environment:
   VIPS_VERSION: "8.15.0"
   VIPS_WARNING: 0
 
+# TODO(kleisauke): Remove when .NET 8.0 is available in AppVeyor
+install:
+  - ps: |
+      Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1' -UseBasicParsing -OutFile "$env:temp\dotnet-install.ps1"
+      & $env:temp\dotnet-install.ps1 -Architecture x64 -Channel 8.0 -InstallDir "$env:ProgramFiles\dotnet"
+
 # Build script
 build_script:
   - ps: .\build.ps1 --package
@@ -21,9 +27,6 @@ skip_tags: true
 pull_requests:
   do_not_increment_build_number: true
 
-# TODO(kleisauke): Remove when .NET 8.0 is available in AppVeyor
-build: off
-
 # Disable automatic tests and deployment
 test: off
 deploy: off