stretch-security buildd builds are now broken (after 2021-10-01; le cert issue?)

[Boyuan Yang <byang@debian.org>]

(Please cc me when needed)

Dear LTS folks,

I'd like to raise the issue that buildd stretch-security builds are now
completely broken after 2021-10-01. For example, see
https://buildd.debian.org/status/architecture.php?a=all&suite=stretch-security
and
https://buildd.debian.org/status/logs.php?pkg=krb5&ver=1.15-1%2Bdeb9u3&arch=all
. The root cause might be
https://lists.debian.org/debian-lts/2021/09/msg00008.html :

+-----------------------------------------------------------------------------
-+
| Update chroot                                                              
|
+-----------------------------------------------------------------------------
-+

Ign:1 https://deb.debian.org/debian stretch InRelease
Ign:2 https://security-master.debian.org/debian-security stretch/updates
InRelease
Err:3 https://deb.debian.org/debian stretch Release
  server certificate verification failed. CAfile: /etc/ssl/certs/ca-
certificates.crt CRLfile: none

Thanks,
Boyuan Yang