As noted in commit 255c715624
`/var/cache/distfiles` is writable by everyone. It is supposed to be
writable only by `root` and by the `abuild` group (in which we put the
`pmos` user already for building packages).
Changes:
* `pmb.build.init()`: make `/var/cache/distfiles` writable only by
members of the `abuild` group (and root)
* Increase workfolder version to 2
* Add migration code that fixes the permissions for existing work
folders
* Refactor the migration code a bit to make this possible
4d8afc4aa5