autopkgtest for src:pam failed with 'su: Permission denied'

Hi,

I get this during autopkgtest before running test:

autopkgtest [13:29:44]: test pam-auth-update: [-----------------------
su: Permission denied
autopkgtest [13:29:46]: test pam-auth-update: -----------------------]
Unexpected error:
Traceback (most recent call last):
  File "/usr/share/autopkgtest/lib/VirtSubproc.py", line 869, in mainloop
    command()
  File "/usr/share/autopkgtest/lib/VirtSubproc.py", line 784, in command
    r = f(c, ce)
        ^^^^^^^^
  File "/usr/share/autopkgtest/lib/VirtSubproc.py", line 737, in cmd_copyup
    copyupdown(c, ce, True)
  File "/usr/share/autopkgtest/lib/VirtSubproc.py", line 621, in copyupdown
    copyupdown_internal(ce[0], c[1:], upp)
  File "/usr/share/autopkgtest/lib/VirtSubproc.py", line 646, in copyupdown_internal
    copyup_shareddir(sd[0], sd[1], dirsp, downtmp_host)
  File "/usr/share/autopkgtest/lib/VirtSubproc.py", line 560, in copyup_shareddir
    shutil.copy(tb, host)
  File "/usr/lib/python3.11/shutil.py", line 419, in copy
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  File "/usr/lib/python3.11/shutil.py", line 256, in copyfile
    with open(src, 'rb') as fsrc:
         ^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/autopkgtest-lxc.yawiehf5/downtmp/pam-auth-update-stdout'
autopkgtest [13:29:47]: ERROR: testbed failure: unexpected eof from the testbed

https://salsa.debian.org/rouca/pam/-/jobs/7893747