Update appveyor.yml

Author: BornToBeRoot

appveyor.yml

@@ -3,6 +3,9 @@ configuration: Release
 platform: x64
 install:
   - git submodule update --init
+  - ps: |
+      Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1' -UseBasicParsing -OutFile "$env:temp\dotnet-install.ps1"
+      & $env:temp\dotnet-install.ps1 -Architecture x64 -Version '10.0.100' -InstallDir "$env:ProgramFiles\dotnet"
 before_build:
   - dotnet restore %APPVEYOR_BUILD_FOLDER%\Source\NETworkManager.sln
   - ps: |