cmd-buildfetch: treat 504 as if the object doesn't exist

The RHCOS builds browser seems to be throwing 504 when the object
doesn't exist:

```
Fetching: <snip>/storage/prod/streams/rhel-10.2/builds/builds.json
Updated builds/builds.json
Traceback (most recent call last):
  File "/usr/lib/coreos-assembler/cmd-buildfetch", line 329, in <module>
    sys.exit(main())
             ~~~~^^
  File "/usr/lib/coreos-assembler/cmd-buildfetch", line 156, in main
    if fetcher.exists(f'{builddir}/{f}'):
       ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/coreos-assembler/cmd-buildfetch", line 251, in exists
    return self.exists_impl(url)
           ~~~~~~~~~~~~~~~~^^^^^
  File "/usr/lib/python3.14/site-packages/tenacity/__init__.py", line 334, in wrapped_f
    return copy(f, *args, **kw)
  File "/usr/lib/python3.14/site-packages/tenacity/__init__.py", line 473, in __call__
    do = self.iter(retry_state=retry_state)
  File "/usr/lib/python3.14/site-packages/tenacity/__init__.py", line 374, in iter
    result = action(retry_state)
  File "/usr/lib/python3.14/site-packages/tenacity/__init__.py", line 396, in <lambda>
    self._add_action_func(lambda rs: rs.outcome.result())
                                     ~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/concurrent/futures/_base.py", line 443, in result
    return self.__get_result()
           ~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/concurrent/futures/_base.py", line 395, in __get_result
    raise self._exception
  File "/usr/lib/python3.14/site-packages/tenacity/__init__.py", line 476, in __call__
    result = fn(*args, **kwargs)
  File "/usr/lib/coreos-assembler/cmd-buildfetch", line 291, in exists_impl
    raise Exception(f"Received rc {r.status_code} for {url}")
Exception: Received rc 504 for <snip>/storage/prod/streams/rhel-10.2/builds/10.2.20260307-0/x86_64/ostree-commit-object
failed to execute cmd-buildfetch: exit status 1
+ rc=1
+ set +x
```

Author: dustymabe

src/cmd-buildfetch

@@ -284,9 +284,11 @@ class HTTPFetcher(Fetcher):
         with requests.head(url) as r:
             if r.status_code == 200:
                 return True
-            # just treat 403 as ENOENT too; this is common for APIs to do (like
+            # Just treat 403 as ENOENT too; this is common for APIs to do (like
             # AWS) and we don't support HTTP basic auth here anyway
-            if r.status_code in [404, 403]:
+            # Lump 504 in here because the RHCOS builds browser/proxy returns a
+            # 504 if the object doesn't exist.
+            if r.status_code in [404, 403, 504]:
                 return False
             raise Exception(f"Received rc {r.status_code} for {url}")