Acquire dotnet-install.ps1 from dot.net (#1849)

Build/build.ps1

@@ -70,6 +70,8 @@ task Build -depends Clean {
   Write-Host -ForegroundColor Green "Found $($script:enabledBuilds.Length) enabled builds"
 
   mkdir "$buildDir\Temp" -Force
+  
+  EnsureDotNetCli
   EnsureNuGetExists
   EnsureNuGetPackage "vswhere" $vswherePath $vswhereVersion
   EnsureNuGetPackage "NUnit.ConsoleRunner" $nunitConsolePath $nunitConsoleVersion
@@ -161,6 +163,19 @@ function NetCliBuild()
   exec { & $script:msBuildPath "/t:build" "/v:$msbuildVerbosity" $projectPath "/p:Configuration=Release" "/p:LibraryFrameworks=`"$libraryFrameworks`"" "/p:TestFrameworks=`"$testFrameworks`"" "/p:AssemblyOriginatorKeyFile=$signKeyPath" "/p:SignAssembly=$signAssemblies" "/p:TreatWarningsAsErrors=$treatWarningsAsErrors" "/p:AdditionalConstants=$additionalConstants" "/p:GeneratePackageOnBuild=$buildNuGet" "/p:ContinuousIntegrationBuild=true" "/p:PackageId=$packageId" "/p:VersionPrefix=$majorWithReleaseVersion" "/p:VersionSuffix=$nugetPrerelease" "/p:AssemblyVersion=$assemblyVersion" "/p:FileVersion=$version" "/m" }
 }
 
+function EnsureDotnetCli()
+{
+  Write-Host "Downloading dotnet-install.ps1"
+
+  # https://stackoverflow.com/questions/36265534/invoke-webrequest-ssl-fails
+  [Net.ServicePointManager]::SecurityProtocol = 'TLS12'
+  Invoke-WebRequest `
+    -Uri "https://dot.net/v1/dotnet-install.ps1" `
+    -OutFile "$buildDir\Temp\dotnet-install.ps1"
+
+  exec { & $buildDir\Temp\dotnet-install.ps1 -Channel $netCliChannel -Version $netCliVersion | Out-Default }
+}
+
 function EnsureNuGetExists()
 {
   if (!(Test-Path $nugetPath))
@@ -194,8 +209,6 @@ function NetCliTests($build)
   $location = "$sourceDir\Newtonsoft.Json.Tests"
   $testDir = if ($build.TestFramework -ne $null) { $build.TestFramework } else { $build.Framework }
 
-  exec { .\Tools\Dotnet\dotnet-install.ps1 -Channel $netCliChannel -Version $netCliVersion | Out-Default }
-
   try
   {
     Set-Location $location