1.3~pre2ubuntu2 regression: trusted.gpg.d not read any more from apt root

The proposed apt is currently stuck (http://people.canonical.com/~ubuntu-archive/proposed-migration/update_excuses.html#apt) as it causes a regression in apport: This causes several failures like

ERROR: test_install_package_from_a_ppa (__main__.T)
Install a package from a PPA.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 462, in update
    pulse_interval)
apt_pkg.Error: W:GPG error: http://ppa.launchpad.net/brian-murray/ppa/ubuntu trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 915A9DFB6AF4FEF1, E:The repository 'http://ppa.launchpad.net/brian-murray/ppa/ubuntu trusty InRelease' is not signed., W:Updating from such a repository can't be done securely, and is therefore disabled by default., W:See apt-secure(8) manpage for repository creation and user configuration details., W:GPG error: http://ddebs.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C8CAB6595FDFF622, E:The repository 'http://ddebs.ubuntu.com trusty Release' is not signed.

This uses apt with a "sandbox" to be able to run apt with normal user privs without having to modify the system.

This can be reproduced using the attached sandbox dir. Unpack it in /tmp, then

    python3 -c 'import apt; c=apt.Cache(rootdir="/tmp/sandbox"); c.update()'

this works fine up to current yakkety (1.3~pre1), but breaks with the -proposed version:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 462, in update
    pulse_interval)
apt_pkg.Error: W:GPG error: http://ddebs.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C8CAB6595FDFF622, E:The repository 'http://ddebs.ubuntu.com trusty Release' is not signed.

But the key is definitively present:

$ gpg --list-keys --keyring /tmp/sandbox/etc/apt/trusted.gpg.d/ddebs.ubuntu.com.gpg
/tmp/sandbox/etc/apt/trusted.gpg.d/ddebs.ubuntu.com.gpg
-------------------------------------------------------
pub 4096R/5FDFF622 2016-03-21 [expires: 2021-03-20]
uid Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: apt 1.3~pre2ubuntu2
ProcVersionSignature: User Name 4.4.0-33.52-generic 4.4.15
Uname: Linux 4.4.0-33-generic x86_64
ApportVersion: 2.20.2-0ubuntu1
Architecture: amd64
Date: Thu Jul 28 11:35:19 2016
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)