In CI, combine `python` and `check-ros2-workspace` jobs

[onkoe]

Because of how many system dependencies ROS 2 requires, our Pixi cache is huuuuuge! It takes a very long time for the runner to download it.

In fact, it's usually about a minute of waiting.

So, we can shave off a minute of used Actions runner time by combining the two jobs into one ci job.

Checklist

Please perform the following steps in the .github/workflows/ci.yml file:

• Rename the check-ros2-workspace job to ci
• Move the stuff in python job both: a) after the Pixi cache restoration, and b) before anything else
  • It needs to be before anything else. That's because we want bad formatting/linting to fail our run before performing any giant 20 minute builds... ;D