Show freed disk space after running `pixi clean cache`

[apcamargo]

Problem description

It would be useful if pixi clean cache reported how much disk space was freed after execution. This kind of feedback helps users understand the impact of the command.

This information is particularly helpful when users are cleaning caches to recover disk space on shared systems or machines with limited storage, where it is important to quickly assess whether the cleanup had a meaningful effect.

As an example, here is the output of uv clean:

$ uv clean
Clearing cache at: /Users/apcamargo/.cache/uv
Removed 866 files (38.7MiB)

[prady0t]

Happy to take this.