Fix Nuke.GlobalTool to target net6.0

Author: matkoch

source/Nuke.GlobalTool/Nuke.GlobalTool.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFrameworks>netcoreapp3.1;net6.0</TargetFrameworks>
     <RollForward>LatestMajor</RollForward>
     <PackAsTool>true</PackAsTool>
     <ToolCommandName>nuke</ToolCommandName>