Show missing image warning at end of build

[andrewbaxter]

Checklist

• [X] I've looked through the issues and pull requests for similar request
• [ ] This feature could be solved with a custom image (optional)

Describe your request

From https://github.com/cross-rs/cross/issues/238#issuecomment-2576741217

Not needed if #1591 is implemented.

Right now there's a warning if the image for the build is missing that it's falling back to the local cargo. The warning is quickly moved off screen for new builds due to all the Compiling... lines making it hard to notice.

Describe why this would be a good inclusion for cross

The issue is cross specific.