Sign the output NuGet packages (#492)

Progress on https://github.com/NuGet/Engineering/issues/2244

.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.ApplicationInsights.JavaScript" version="1.0.17-build01715" />
   <package id="Microsoft.CodeAnalysis.BinSkim" version="1.3.6" />
   <package id="xunit.runner.console" version="2.1.0" />

build.ps1

@@ -9,7 +9,7 @@ param (
     [string]$SemanticVersion = '1.0.0-zlocal',
     [string]$Branch,
     [string]$CommitSHA,
-    [string]$BuildBranch = 'b5f9d1c89da96c462935e2195ceb00e69287b93e'
+    [string]$BuildBranch = '2d8feecabe3aeaed7f5b4d50b9be78c94faf39ec'
 )
 
 # For TeamCity - If any issue occurs, this script fails the build. - By default, TeamCity returns an exit code of 0 for all powershell scripts, even if they fail
@@ -122,6 +122,11 @@ 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