Sign the NuGet packages produced by the build (#6999)

Progress on NuGet/Engineering#2244

Author: joelverhagen

.nuget/packages.config

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
+  <package id="MicroBuild.Core" version="0.3.0" />
   <package id="Microsoft.CodeAnalysis.BinSkim" version="1.3.6" />
   <package id="xunit.runner.console" version="2.1.0" />
   <package id="xunit.runner.visualstudio" version="2.1.0" />

build.ps1

@@ -10,7 +10,7 @@ param (
     [string]$PackageSuffix,
     [string]$Branch,
     [string]$CommitSHA,
-    [string]$BuildBranch = '5fd8377a9abf3ff411918dbb973948a6677432db'
+    [string]$BuildBranch = '2d8feecabe3aeaed7f5b4d50b9be78c94faf39ec'
 )
 
 Set-StrictMode -Version 1.0
@@ -99,6 +99,11 @@ Invoke-BuildStep 'Creating artifacts' { `
 } `
 -ev +BuildErrors
 
+Invoke-BuildStep 'Signing the packages' {
+    Sign-Packages -Configuration $Configuration -BuildNumber $BuildNumber -MSBuildVersion "15" `
+} `
+-ev +BuildErrors
+
 Trace-Log ('-' * 60)
 
 ## Calculating Build time