Update dependencies from https://github.com/dotnet/arcade build 20200819.21 (#8234)

[master] Update dependencies from dotnet/arcade
- Updates:
  - Microsoft.DotNet.Build.Tasks.Installers: from 5.0.0-beta.20417.6 to 5.0.0-beta.20419.21
  - Microsoft.DotNet.Arcade.Sdk: from 5.0.0-beta.20417.6 to 5.0.0-beta.20419.21

eng/common/post-build/publish-using-darc.ps1

@@ -1,12 +1,13 @@
 param(
   [Parameter(Mandatory=$true)][int] $BuildId,
+  [Parameter(Mandatory=$true)][int] $PublishingInfraVersion,
   [Parameter(Mandatory=$true)][string] $AzdoToken,
   [Parameter(Mandatory=$true)][string] $MaestroToken,
   [Parameter(Mandatory=$false)][string] $MaestroApiEndPoint = 'https://maestro-prod.westus2.cloudapp.azure.com',
   [Parameter(Mandatory=$true)][string] $WaitPublishingFinish,
-  [Parameter(Mandatory=$true)][string] $EnableSourceLinkValidation,
-  [Parameter(Mandatory=$true)][string] $EnableSigningValidation,
-  [Parameter(Mandatory=$true)][string] $EnableNugetValidation,
+  [Parameter(Mandatory=$false)][string] $EnableSourceLinkValidation,
+  [Parameter(Mandatory=$false)][string] $EnableSigningValidation,
+  [Parameter(Mandatory=$false)][string] $EnableNugetValidation,
   [Parameter(Mandatory=$true)][string] $PublishInstallersAndChecksums,
   [Parameter(Mandatory=$false)][string] $ArtifactsPublishingAdditionalParameters,
   [Parameter(Mandatory=$false)][string] $SigningValidationAdditionalParameters
@@ -14,7 +15,8 @@ param(
 
 try {
   . $PSScriptRoot\post-build-utils.ps1
-  . $PSScriptRoot\..\darc-init.ps1
+  # Hard coding darc version till the next arcade-services roll out, cos this version has required API changes for darc add-build-to-channel
+  . $PSScriptRoot\..\darc-init.ps1 -darcVersion "1.1.0-beta.20418.1"
 
   $optionalParams = [System.Collections.ArrayList]::new()
 
@@ -49,12 +51,13 @@ try {
   }
 
   & darc add-build-to-channel `
-	--id $buildId `
-	--default-channels `
-	--source-branch master `
-	--azdev-pat $AzdoToken `
-	--bar-uri $MaestroApiEndPoint `
-	--password $MaestroToken `
+  --id $buildId `
+  --publishing-infra-version $PublishingInfraVersion `
+  --default-channels `
+  --source-branch master `
+  --azdev-pat $AzdoToken `
+  --bar-uri $MaestroApiEndPoint `
+  --password $MaestroToken `
 	@optionalParams
 
   if ($LastExitCode -ne 0) {