Updated linux shell scripts for UBT in NetCore, opt in by setting UE_USE_DOTNET=1, note that due to output path differences a lot of tooling will break when enabling it.

Added download.sh scripts to download new versions of dotnet sdk to inline for Linux and Mac.

#rb ben.marsh

[CL 14445358 by Joakim Lindqvist in ue5-main branch]

Engine/Build/BatchFiles/Linux/SetupEnvironment.sh

@@ -23,6 +23,11 @@ do
             source "$2/SetupMono.sh" "$2"
             shift
             ;;
+        -dotnet) 
+            # echo "setting up dotnet"
+            source $2/SetupDotnet.sh "$2"
+            shift
+            ;;
         -*) echo "bad argument $1"
             ;;
     esac