Update PS build script to set env:NUGET_PACKAGES to support custom package installation directory.

PowerShellGitHubDev.psm1

@@ -66,6 +66,11 @@ function Start-PSBuild {
         }
     }
 
+	# use custom package store - this value is also defined in nuget.config under config/repositoryPath
+	# dotnet restore uses this value as the target for installing the assemblies for referenced nuget packages.
+	# dotnet build does not currently consume the  config value but will consume env:NUGET_PACKAGES to resolve these dependencies
+	$env:NUGET_PACKAGES="$PSScriptRoot\Packages"
+
     # verify we have all tools in place to do the build
     $precheck = precheck 'dotnet' "Build dependency 'dotnet' not found in PATH! See: https://dotnet.github.io/getting-started/"
     if ($FullCLR) {