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.

Bug: 431628