try fix ps1 script

Author: jogibear9988

nuget/pack.ps1

@@ -13,12 +13,12 @@ $content = $content -replace '\$releasenotes\$',$env:APPVEYOR_REPO_COMMIT_MESSAG
 
 $content | Out-File $root\nuget\WpfDesigner.compiled.nuspec
 
-$root\nuget\nuget.exe pack $root\nuget\WpfDesigner.compiled.nuspec
+& $root\nuget\nuget.exe pack $root\nuget\WpfDesigner.compiled.nuspec
 
 $content = (Get-Content $root\NuGet\WpfDesigner.ExpressionBlendInteractionAddon.nuspec) 
 $content = $content -replace '\$version\$',$versionStr
 $content = $content -replace '\$releasenotes\$',$env:APPVEYOR_REPO_COMMIT_MESSAGE
 
 $content | Out-File $root\nuget\WpfDesigner.ExpressionBlendInteractionAddon.compiled.nuspec
 
-$root\nuget\nuget.exe pack $root\nuget\WpfDesigner.ExpressionBlendInteractionAddon.compiled.nuspec
+& $root\nuget\nuget.exe pack $root\nuget\WpfDesigner.ExpressionBlendInteractionAddon.compiled.nuspec