[CI] Always run PRERELEASE tests

Our current CI configuration only ran prerelease tests for ROS2. We recently had a CMake breakage for Python2

Cf.
ros/rosdistro#34135 (comment)
stack-of-tasks/eigenpy-ros-release#4

Author: wxmerkt

.github/workflows/ros_ci.yml

@@ -11,14 +11,16 @@ jobs:
     strategy:
       matrix:
         env:
-          - {ROS_DISTRO: melodic}
-          - {ROS_DISTRO: noetic, BUILDER: catkin_tools}
+          - {ROS_DISTRO: melodic, PRERELEASE: false}
+          - {ROS_DISTRO: noetic}
           - {ROS_DISTRO: foxy}
           - {ROS_DISTRO: galactic}
-          - {ROS_DISTRO: foxy, PRERELEASE: true}
-          - {ROS_DISTRO: galactic, PRERELEASE: true}
+          - {ROS_DISTRO: rolling}
+          - {ROS_DISTRO: humble}
     env:
       CCACHE_DIR: /github/home/.ccache # Enable ccache
+      PRERELEASE: true
+      BUILDER: colcon
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2