Fix for enabling external interpreter if we don't have internal interpreter does nothing (#16331)

If you inspect the logic in GHC.Driver.Pipeline there is this snippet.

        -- #16331 - when no "internal interpreter" is available but we            
        -- need to process some TemplateHaskell or QuasiQuotes, we automatically       
        -- turn on -fexternal-interpreter.                                        
        dflags2 = if not internalInterpreter                                      
                  then gopt_set dflags1 Opt_ExternalInterpreter                   
                  else dflags1 

but then if you look at the definition of internalInterpreter

       internalInterpreter = not (gopt Opt_ExternalInterpreter lcl_dflags) 

So we only enable this extension if it is already set.