Wipe existing signatures when creating partition table

As the sfdisk man page says on the option '--wipe' that specifies whether or not to wipe signatures in order to avoid collisions:

When this option is not given, the default is auto, in which case signatures are wiped only when in interactive mode

but sfdisk is not run in interactive mode here, since stdin does not refer to a terminal, and the sfdisk man page says:

It [i.e. sfdisk] runs in interactive mode if executed on terminal (stdin refers to a terminal).

Therefore, explicitly pass the '--wipe=always' option to sfdisk so that old signatures are wiped when a new partition table is created.

This e.g. fixes https://bugs.kde.org/show_bug.cgi?id=431628 ("partitionmanager does not wipe old signatures when creating a new partition table")