Added pip installation to core tutorials. (#204)

Author: FranFin

_includes/first_application_common/build_system.md

@@ -22,6 +22,9 @@ mkdir microros_ws
 cd microros_ws
 git clone -b $ROS_DISTRO https://github.com/micro-ROS/micro-ros-build.git src/micro-ros-build
 
+# Install pip
+sudo apt-get install python3-pip
+
 # Update dependencies using rosdep
 sudo apt update && rosdep update
 rosdep install --from-path src --ignore-src -y