Support saving package build buildinfo files to support reproducible builds

[Philip Roche]

In the Canonical Public Cloud team we have been approached by some of our Google colleagues to help in supporting reproducible builds for packages in the Ubuntu archive.

See below for more information on reproducible builds @

* https://reproducible-builds.org/
* https://wiki.debian.org/ReproducibleBuilds/
* https://wiki.debian.org/ReproducibleBuilds/BuildinfoFiles
* https://tests.reproducible-builds.org/debian/reproducible.html

The .buildinfo file required to support reproducible builds are present in Debian but not in Ubuntu and our Google colleagues are consuming these as part of their package validation work.

However they are present in the Ubuntu package build logs but not saved. We have created https://github.com/canonical/ubuntu-package-buildlog-info as an interim tool to extract these, but we want the buildinfo files to be saved properly as part of a build.

Canonical folks have discussed this in person at Riga November 2023 sprint and agreed that the missing saving and publication of these .buildinfo files should be treated as a bug. This is that bug.

The goal would be to:

 * Save the buildinfo files as part of package build and serve as build artifacts, preferably surfaced via the launchpad API too.
 * Decide on a plan to publish these buildinfo in some way via the archive or otherwise.

[Andrew Cloke (andrew-cloke)]

From internal discussions, it appears that the build info files are being exported from the LP builds, but they are not currently being exposed.

For reference, Colin Watson's comments on the following MP are also relevant: https://code.launchpad.net/~tsimonq2/launchpad/+git/launchpad/+merge/448527 .

[Philip Roche (philroche)]

This is a duplicate of https://bugs.launchpad.net/launchpad/+bug/1686242 which is now Fix Released. Marking this as Fix Released now too.