sbuild --chroot-mode=unshare -d $suite{,-security,-proposed} magic for Ubuntu

[Julian Andres Klode]

Hey folks,

(just in proposed but don't want to forget to announce it :D)

sbuild recently gained the magic to automatically bootstrap
tarballs for the unshare backend using mmdebstrap. No more
schroots to manage, everything is magical!

I have just uploaded a new sbuild that makes this feature
work correctly for Ubuntu. If you set

    $chroot_mode = "unshare";
    $unshare_mmdebstrap_keep_tarball = 1;

(the second one is optionally and makes sbuild cache the tarballs
 in ~/.cache/sbuild; refreshing them after 7 days)

in your sbuild configuration (or pass them on the command-line), then

* `sbuild -d noble` will build the package for noble, with updates
   enabled
* `sbuild -d noble-proposed` builds with proposed enabled too
* `sbuild -d noble-security` builds for the security pocket
* `sbuild -d noble-backports` builds with backports enabled too (but
  I think it's still pinned down, I assume you want to use a different
  solver here to not pull in all backported packages?)

We enable the `main` and `universe` components by default. Should
we perhaps enable `restricted` and `multiverse` too? There's no
easy way otherwise to override.

I could not get the tests passing locally so far, but I saw no
regression and the changes seem unlikely to cause any, so I uploaded
in the hopes they infrastructure fairs better.

I've now managed to run them and there is failure related to
cross-bootstrap (yes sbuild --arch=armhf -d plucky also works);
I'll dig deeper and hopefully fix that too.

You'll need to install mmdebstrap for this. Once it has landed
I'll try to make the case for the mmdebstrap MIR again.
-- 
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