plugin: in release, create a portable pdb file

The nuget symbols server only supports portable pdbs. This results in
"Symbols package publishing failed." when uploading the .snupkg file

Author: JohnTheGr8

Flow.Launcher.Plugin/Flow.Launcher.Plugin.csproj

@@ -43,7 +43,7 @@
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
-    <DebugType>pdbonly</DebugType>
+    <DebugType>portable</DebugType>
     <Optimize>true</Optimize>
     <OutputPath>..\Output\Release\</OutputPath>
     <DefineConstants>TRACE</DefineConstants>