RHEL CI jobs have many colcon warnings

[clalancette]

If you look at any of the nightly RHEL jobs (e.g. https://ci.ros2.org/view/nightly/job/nightly_linux-rhel_release/1758/consoleFull), you'll see a ton of warnings coming from colcon that look like:

[700.684s] colcon.colcon_core.extension_point ERROR Entry point 'colcon_core.shell.bash' is declared multiple times, 'colcon_bash.shell.bash:BashShell' overwriting 'colcon_bash.shell.bash:BashShell'

After talking to @cottsay about it, it looks like this is probably a problem of installing colcon to a particular path without removing other versions. We should fix this bug, which will reduce the overall size of the RHEL job logs and make it easier to look at.