Don't measure wheel size with du
The `du` tool estimates disk usage, so filesystem settings
(block size, compression, holes, etc.) might change the result.
Use `stat` instead for a more reproducible result.
Note that I've changed 900 (KiB) to 900000 (bytes) which is in fact not correct
(900 KiB is 921600 bytes),
but I assumed a human-readable round number works better here.
I've updated the comment to say kB instead of KiB.
Fixes https://bugzilla.redhat.com/2143393