Refactor examples to allow passing pkgs as an argument

This will be useful in the next commit, where we integrate the example
into flake.nix.

examples/ros2-basic.nix

@@ -1,6 +1,7 @@
 # Environment containing basic ROS2 tools
 
-with import ../. {};
+{ pkgs ? import ../. {} }:
+with pkgs;
 with rosPackages.humble;
 
 mkShell {

examples/turtlebot3-gazebo.nix

@@ -2,7 +2,8 @@
 # roslaunch turtlebot3_gazebo turtlebot3_world.launch
 # roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch
 
-with import ../. {};
+{ pkgs ? import ../. {} }:
+with pkgs;
 with rosPackages.noetic;
 with pythonPackages;