tests: Add Debian 11/bullseye security archive signing key
Tests that install packages are failing due to repos/packages that are signed
with this key.
```console
$ wget https://ftp-master.debian.org/keys/archive-key-11-security.asc
--2025-06-17 14:36:04-- https://ftp-master.debian.org/keys/archive-key-11-security.asc
Resolving ftp-master.debian.org (ftp-master.debian.org)... 192.91.235.231
Connecting to ftp-master.debian.org (ftp-master.debian.org)|192.91.235.231|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 11873 (12K) [application/pgp-keys]
Saving to: ‘archive-key-11-security.asc’
archive-key-11-security.asc 100%[=================>] 11.59K --.-KB/s in 0.002s
2025-06-17 14:36:05 (6.64 MB/s) - ‘archive-key-11-security.asc’ saved [11873/11873]
$ sha256sum archive-key-11-security.asc
716e79393c724d14ecba8be46e99ecbe1b689f67ceff3cb3cab28f6e69e8b8b8 archive-key-11-security.asc
$ cp archive-key-11-security.asc \
~/src/mitogen/tests/image_prep/roles/package_manager/files/debian-archive-bullseye-security-automatic.asc
```
|
Alex Willmer