Private PPA has a pending publication for hours

relevant logfile ouput

```
2023-04-24 09:45:08 ERROR Unhandled exception
Traceback (most recent call last):
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/services/scripts/base.py", line 93, in log_unhandled_exceptions_func
    return func(self, *args, **kw)
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/services/scripts/base.py", line 371, in lock_and_run
    self.run(use_web_security=use_web_security, isolation=isolation)
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/services/scripts/base.py", line 93, in log_unhandled_exceptions_func
    return func(self, *args, **kw)
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/services/scripts/base.py", line 321, in run
    self.main()
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/archivepublisher/scripts/publishdistro.py", line 710, in main
    archive_id, reset_store=reset_store_between_archives
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/archivepublisher/scripts/publishdistro.py", line 582, in processArchive
    self.publishArchive(archive, publisher)
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/archivepublisher/scripts/publishdistro.py", line 541, in publishArchive
    archive, dirty_suite
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/archivepublisher/scripts/publishdistro.py", line 467, in syncOVALDataFilesForSuite
    staged_oval_data_dir, dest_dir
  File "/srv/launchpad.net/production/launchpad-rev-90e2150bef31b411d7bae5c4032c7e320fcaaec8/lib/lp/archivepublisher/scripts/publishdistro.py", line 441, in synchronizeSecondDirectoryWithFirst
    os.unlink(str(second_dir / file))
IsADirectoryError: [Errno 21] Is a directory: '/srv/launchpad.net/ppa-archive/cloud-images/gke-1.22/ubuntu/dists/focal/main/oval/by-hash'
2023-04-24 09:45:08 INFO OOPS-22780febd6b786ee481d93d0f31b7db0
+ /srv/launchpad.net/production/launchpad/cronscripts/publishing/../../scripts/publish-distro.py -v --private-ppa -d ubuntu
lockfile: Sorry, giving up on "/srv/launchpad.net/ppa-archive/.lock"
++ echo 'Could not claim lock file.'
Could not claim lock file.
++ exit 1
```

=> regression from recent changes for OVAL data distribution

Cowboy deployed at UTC 1146 by @ballot.

```
diff --git a/lib/lp/archivepublisher/scripts/publishdistro.py b/lib/lp/archivepublisher/scripts/publishdistro.py
index 754c0bf3144..ea515f17dd7 100644
--- a/lib/lp/archivepublisher/scripts/publishdistro.py
+++ b/lib/lp/archivepublisher/scripts/publishdistro.py
@@ -438,7 +438,10 @@ def synchronizeSecondDirectoryWithFirst(self, first_dir, second_dir):
             copy(str(first_dir / file), str(second_dir))

         for file in files_to_delete:
- os.unlink(str(second_dir / file))
+ try:
+ os.unlink(str(second_dir / file))
+ except IsADirectoryError:
+ pass

         return bool(files_to_copy) or bool(files_to_delete)
```

Last occurrence of the issue before the cowboy OOPS-1f4f70ff67b93d667fcfc4680644a7f9

Cowboy has been applied, and the publisher resumed work.