Update dotnet-install.ps1 URL.

This is taken from https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-install-script#recommended-version.

Author: bgrainger

appveyor.yml

@@ -8,8 +8,7 @@ environment:
 cache:
   - '%USERPROFILE%\.nuget\packages -> **\*.csproj'
 install:
-  # The following can be used to install a custom version of .NET Core
-  - ps: Invoke-WebRequest -Uri "https://raw.githubusercontent.com/dotnet/cli/master/scripts/obtain/dotnet-install.ps1" -OutFile "install-dotnet.ps1"
+  - ps: Invoke-WebRequest -Uri "https://dot.net/v1/dotnet-install.ps1" -OutFile "install-dotnet.ps1"
   - ps: .\install-dotnet.ps1 -Version 3.1.301 -InstallDir "dotnetcli"
   - ps: .\install-dotnet.ps1 -Version 5.0.100-preview.6.20318.15 -InstallDir "dotnetcli"
 build_script: