coreutils: avoid prompt on rebuild

The mode bits of coreutils installed binaries are 555. As a result, on
rebuild mv prompts the user to confirm overwrite of non writable binaries.
Force overwrite to skip this prompt.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>