setup-disk: set swap mount point to "none" to avoid creating /swap

From the commit message:

Previously, the swap mount point was set to "swap". This works, but inadvertently creates the directory /swap when booting. According to fstab(5), the mount point should be set to "none" for swap.

I didn't test this properly, I only checked that if I manually edit /etc/fstab after installing then the next boot works (swap is enabled, /swap doesn't exist, /none doesn't exist).