extraPostVM docs: recommendation doesn't work, would be potentially destructive if it did

[colemickens]

1. The impure script uses $PWD to build the disk image.
2. There's no protections for an existing disk image in "$out".
3. The option documentation for extraPostVM:
   i. doesn't work, you can't glob in a script like that
   ii. if it did work, would affect ANY .raw files in $PWD/"$out" (which could be bad!)
   iii. is just a bit unfortunate, it would be a great place to show the user how to actually get the disk image name

I think when I looked the other day, the disk image name isn't straight forward to recompose and isn't stashed. So maybe a way to fix this is to just move the disk name to an exported system.build.disko... whatever attribute, and then use it. Then the user can use that attr to reference the image path in extraPostVM.