BUG when dealing with cached failure with storage service enabled

https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/3179148965

```
[09:39:01][00:00:00][e7beaa5d][ build:components/vulkan-validation-layers.bst] BUG Build

An unhandled exception occured:

Traceback (most recent call last):
File "/usr/lib/python3.10/site-packages/buildstream/_scheduler/jobs/job.py", line 441, in child_action
result = self.child_process() # pylint: disable=assignment-from-no-return
File "/usr/lib/python3.10/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 92, in child_process
return self._action_cb(self._element)
File "/usr/lib/python3.10/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 57, in _assemble_element
element._assemble()
File "/usr/lib/python3.10/site-packages/buildstream/element.py", line 1643, in _assemble
with open(log_path, encoding="utf-8") as log_file:
FileNotFoundError: [Errno 2] No such file or directory: '/builds/freedesktop-sdk/freedesktop-sdk/cache/buildstream/cas/objects/51/6d093b37db81bb547b37b33e9930587a69b35cf18a5d4b9e8a748a7305073a'
```

Start with the linked GitLab job and the traceback through buildstream/_scheduler/jobs/job.py, elementjob.py, buildqueue.py, and element.py. Reproduce the cached failure with the storage service enabled and verify that a missing cached object no longer causes an unhandled FileNotFoundError.

Written by the indexing model from the issue text.