Enable custom dotnet for appveyor (#931)

* Enable dotnet channel for appveyor

* Second try

* Third try

appveyor.yml

@@ -2,13 +2,13 @@ version: '{build}-{branch}'
 pull_requests:
   do_not_increment_build_number: true
 skip_branch_with_pr: true
-image:
-- Visual Studio 2017
-- Visual Studio 2017 Preview
+image: Visual Studio 2017
 configuration: Release
 clone_depth: 10
 environment:
+  DOTNET_CHANNEL: 2.1
   DOTNET_CLI_TELEMETRY_OPTOUT: true
+  DOTNET_INSTALL_DIR: tools\dotnet
   DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
   NET_CORE_VERSION: netcoreapp2.1
   NET_FRAMEWORK_VERSION: net472
@@ -32,6 +32,11 @@ install:
     if ($LastExitCode -ne 0) {
         throw "Last command failed."
     }
+
+    if ($env:DOTNET_CHANNEL) {
+        &([scriptblock]::Create((Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1'))) -Channel "$env:DOTNET_CHANNEL" -InstallDir "$env:DOTNET_INSTALL_DIR" -NoPath
+        $env:Path = "$pwd\$env:DOTNET_INSTALL_DIR;$env:Path"
+    }
 before_build:
 - pwsh: >-
     Set-StrictMode -Version Latest
@@ -41,6 +46,11 @@ before_build:
     $ProgressPreference = 'SilentlyContinue'
 
 
+    if (Test-Path -Path "$env:DOTNET_INSTALL_DIR" -PathType Container) {
+        $env:Path = "$pwd\$env:DOTNET_INSTALL_DIR;$env:Path"
+    }
+
+
     dotnet --info
 
 
@@ -56,6 +66,11 @@ build_script:
     $ProgressPreference = 'SilentlyContinue'
 
 
+    if (Test-Path -Path "$env:DOTNET_INSTALL_DIR" -PathType Container) {
+        $env:Path = "$pwd\$env:DOTNET_INSTALL_DIR;$env:Path"
+    }
+
+
     Push-Location -Path ASF-ui
 
 
@@ -93,6 +108,11 @@ test_script:
     $ProgressPreference = 'SilentlyContinue'
 
 
+    if (Test-Path -Path "$env:DOTNET_INSTALL_DIR" -PathType Container) {
+        $env:Path = "$pwd\$env:DOTNET_INSTALL_DIR;$env:Path"
+    }
+
+
     dotnet test ArchiSteamFarm.Tests -c "$env:CONFIGURATION" -f "$env:NET_CORE_VERSION" -o 'out\source' /nologo
 
 
@@ -108,6 +128,11 @@ after_test:
     $ProgressPreference = 'SilentlyContinue'
 
 
+    if (Test-Path -Path "$env:DOTNET_INSTALL_DIR" -PathType Container) {
+        $env:Path = "$pwd\$env:DOTNET_INSTALL_DIR;$env:Path"
+    }
+
+
     $PublishBlock = {
         param($variant)