Add debug logging to saving of user profiles

We have recently observed CI failures such as
```
error: Failed to create sync archive [1]: tar:
var/check_mk/web/agent_registrar/automation.secret:
file changed as we read it
```

These failures seem to be caused by some other process editing the profile files
of non-exisiting users (the `agent_registrar` user doesn't exist anymore when
the failure happens). The `save_users` function might be the call site causing
the issue. Since we are currently out of other ideas, we add some debug logging
to verify this assumption in case the failure re-occurs.

Change-Id: Ie42855a4e5aafaf7e6f791e65ca07ad646df8d41

Author: jherbel

cmk/gui/userdb/store.py

@@ -36,6 +36,7 @@
 from cmk.gui.hooks import request_memoize
 from cmk.gui.htmllib.html import html
 from cmk.gui.i18n import _
+from cmk.gui.log import logger
 from cmk.gui.logged_in import LoggedInUser, save_user_file
 from cmk.gui.type_defs import (
     SessionInfo,
@@ -359,6 +360,8 @@ def split_dict(d: Mapping[str, Any], keylist: list[str], positive: bool) -> dict
 
 
 def save_users(profiles: Users, now: datetime) -> None:
+    logger.debug(f"Saving the profiles of the following users: {', '.join(sorted(profiles))}")
+
     write_contacts_and_users_file(profiles)
 
     # Execute user connector save hooks