Upstream changes broke the test-humble-garden CI workflow

[M1chaelM]

Upstream changes to ros_gz added a dependency that breaks our build. See discussion in #658. In addition, the github action checkout v2.4 has been deprecated due to deprecation of node12, so this needs to be upgraded to v3.0. Unfortunately, 3.0 does not support Dockerfiles that run as non-root users (and in general github actions do not support this), so it is incompatible with our test image.

[M1chaelM]

CI has been restored by adding the missing dependency to the test image and by #658, which upgrades our workflow to use checkout v3, and overrides the user to run as root.

Two further changes would improve our CI:

• Until github actions supports running as a non-root user, the Docker image should not create a user, thereby eliminating the need to override it. I think we can instead create the user dynamically using github actions during the test process.
• We should either use rosdep or change our install process to use debs for gazebo sim and ros_gz so upstream changes don't break our build. Since we no longer need to build from source, I think switching to debs makes the most sense.

[M1chaelM]

#663 switches over to using debs, so changes in dependencies should now be handled for us automatically. This issue is therefore hereby resolved.