Fix installation for read-only files.

On Windows, we must explicitly add write permissions to the target
file before removing it, as the operation will fail if the file is not
writable.

Task-number: QBS-475
Change-Id: Ide40cc4310e327265b0a5c08f4935866762797b9
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>