Output progress information when builds are being uploaded to the buildd manager

[Iain Lane]

At the end of a build, the following lines are output

RUN: /usr/share/launchpad-buildd/bin/in-target scan-for-processes --backend=lxd --series=focal --arch=arm64 LIVEFSBUILD-210538
Scanning for processes to kill in build LIVEFSBUILD-210538

but after the "Scanning for process..." line, and before Launchpad says it's uploading the build, the build is actually being uploaded to the buildd manager.

There's no indication that this is taking place though. I'm wondering if Launchpad could print that it's doing some uploading. Ideally every <interval> it'd update saying that the upload is still going and how far along it is.

Context: during the Focal beta release we had some very poor performance here, and we were unable to see what was happening with our livefs builds. We were eventually able to find out the progress after Canonical IS intervened and gave us a file listing showing how fast the files were growing. If the build had been outputting status we'd have have better visibility so could have self-served a bit more.