Use launch args as variables in ROS2 launch file

edit

I would like to start a launch file with ros2 launch and pass it arguments that are used in the launch file, like for starting a node in debug mode or starting extra nodes if running in simulation. I hope to achieve something like this from ROS1:

<arg name="debug" default="false" />
<arg unless="$(arg debug)" name="launch_prefix" value="" />
<arg if="$(arg debug)" name="launch_prefix" value="gdb --ex run --args" />

I have seen good examples for passing launch arguments through to node parameters using launch.actions.DeclareLaunchArgument but can't figure out using the launch args inside of the launch file itself. I am using Foxy on Ubuntu 20. This is the minimal launch file I am trying to get working

import os
import yaml
from launch import LaunchDescription
from launch_ros.actions import Node
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration

def generate_launch_description():
    debug_arg = DeclareLaunchArgument('debug', default_value='false')
    debug_prefix = 'xterm -e gdb --args' if LaunchConfiguration('debug')=='true' else ''

    talker_node = Node(
        package='cpp_pubsub',
        executable='talker',
        prefix=[debug_prefix],
        output='screen'
    )

    listener_node = Node(
        package='cpp_pubsub',
        executable='listener',
        output='screen'
    )

    return LaunchDescription([ debug_arg, talker_node, listener_node ])

What I hope to be able to do eventually is ros2 launch package file debug:='true' or similiar