fix: btrfs volumes can not be formatted

[apricote]

We support btrfs volumes since #200, but it looks like the formatting was not added in #279 when we changed the way volumes are formatted.

We need to add a case for formatting btrfs volumes here and to the e2e test configuration to make sure that it actually works and we do not accidentally break it in the future.

Originally posted by @felixscheinost in #200 (comment)

[felixscheinost]

Thank you for opening the issue! I'm curious to see what the e2e tests say. Could be that this is an easy fix, right?

[apricote]

Yea, btrs-progs are already installed, so adding it back in should be ~3 lines and a quick sanity check that it actually works.

[apricote]

Fixed in #424