Use appveyor yml back for build.

appveyor.yml

@@ -2,5 +2,10 @@ clone_depth: 1
 version: '{branch}-{build}'
 image: Visual Studio 2019
 test: off
-build_script:
+before_build:
-  - cmd: PowerShell -Version 2.0 .\build.ps1
+  - ps: dotnet --info # Useful when version mismatch between CI and local
+  - ps: nuget restore -verbosity quiet # Only nuget.exe knows both new (.NET Core) and old (Xamarin) projects
+build:
+  project: osu.sln
+  parallel: true
+  verbosity: minimal