Fix switching between earth/mars in atmosphere example (#24349)

# Objective

The keys for switching between earth and mars atmospheres in the
atmosphere example currently do nothing. This change restores the
functionality.

I'm guessing the issue is from the Atmosphere component changing from
using `Transform` to `GlobalTransform` directly in
`Atmosphere::set_default_transform`. The example is expecting to iterate
over a `Transform`, never finds one, and thus never changes the
atmosphere.

## Solution

Change the example to modify `GlobalTransform` instead of `Transform`. 

It may be preferable to modify the example to insert a `Transform` in
set-up. Let me know if this is the case and i'll update the PR.

## Testing

- Tested by running the atmosphere example, hitting the 3 and 4 keys and
seeing sufficiently earth and mars like atmospheres, as well as messages
in the console.