Sbuild task: No log is kept if build-deps aren't installable

https://debusine.debian.net/work-request/56/ failed to install its build-deps. The only log we have is very unhelpful:

cmd: sbuild --no-clean --arch-any --arch-all --no-source --arch=amd64 --dist=trixie --chroot-mode=unshare --chroot=/var/lib/debusine/worker/system-images/108/system.tar.xz --extra-package=/tmp/debusine-fetch-exec-upload-vf6z9q80/dh-python_6.20231224+noflit_all.deb --extra-package=/tmp/debusine-fetch-exec-upload-vf6z9q80/pybuild-plugin-autopkgtest_6.20231224+noflit_all.deb --extra-package=/tmp/debusine-fetch-exec-upload-vf6z9q80/pybuild-plugin-pyproject_6.20231224+noflit_all.deb /tmp/debusine-fetch-exec-upload-vf6z9q80/pygmsh_7.1.17-4.dsc
output (contains stdout and stderr):
E: Package build dependencies not satisfied; skipping

aborted: False
returncode: 3

Files in working directory:
pygmsh_7.1.17-4_amd64-2024-03-09T18:55:46Z.build
pygmsh_7.1.17-4_amd64.build
--------------------

We should keep the sbuild log, even if it failed.

It looks like this is a logic error in Task._execute. If task_succeeded returns false, we never get around to uploading artifacts, and the log is an artifact.