Update ros2-basic example to include ros-core and colcon

Previously, ros-core was not included presumably due to its dependency
on ros2doctor, which depended on ifcfg package unavailable in nixpkgs.
Since ros2cli version 0.17.0 (2022-01-25) ifcfg is no longer necessary
as it was replaced by psutils [1]. This means that we can use ros-core
without problems instead of manually specifying its dependencies such
ros-environment, ros2topic, etc.

In addition to ros-core, colcon is a basic tool for ROS 2. Include it
in the example too.

[1]: cc5d6b1f9b

examples/ros2-basic.nix

@@ -8,9 +8,8 @@ mkShell {
   nativeBuildInputs = [
     (buildEnv {
       paths = [
-        ros-environment
-        ros2topic
-        ros2node
+        ros-core
+        colcon
         geometry-msgs
       ];
     })