An unhandled exception occured, when passing bogus input to bst

See elements is not an absolute path
```
# File generated by convert.py

build-depends:
- freedesktop-sdk.bst:components/gtk3.bst
depends: []
sources: []
kind: script
config:
commands:
- install -m755 install-licenses.sh /usr/bin
variables:
cwd: elements

```

Error message:
```
[00:00:00][7aac01d5][ build:licence-extractor.bst ] BUG Build

An unhandled exception occured:

Traceback (most recent call last):
File "/usr/lib64/python3.13/site-packages/buildstream/_scheduler/jobs/job.py", line 350, in child_action result = self.child_process() # pylint: disable=assignment-from-no-return
File "/usr/lib64/python3.13/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 81, in child_process return self._action_cb(self._element)
~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
File "/usr/lib64/python3.13/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 55, in _assemble_element element._assemble()
~~~~~~~~~~~~~~~~~^^
File "/usr/lib64/python3.13/site-packages/buildstream/element.py", line 1695, in _assemble
self.__configure_sandbox(sandbox)
~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^ File "/usr/lib64/python3.13/site-packages/buildstream/element.py", line 2681, in __configure_sandbox
self.configure_sandbox(sandbox) ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/usr/lib64/python3.13/site-packages/buildstream/scriptelement.py", line 200, in configure_sandbox
sandbox.set_work_directory(self.__cwd)
~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
File "/usr/lib64/python3.13/site-packages/buildstream/sandbox/sandbox.py", line 149, in set_work_directory
assert directory.startswith("/"), "The working directory must be an absolute path"
~~~~~~~~~~~~~~~~~~~~^^^^^
AssertionError: The working directory must be an absolute path

```

One would expect only an error message (or alternatively making the path absolute relative to where bst is run from or where project.conf is.

Start with buildstream/scriptelement.py at configure_sandbox() and buildstream/sandbox/sandbox.py at set_work_directory(), following how the relative cwd from the element configuration reaches the assertion. Done when bogus input produces a clear user-facing error instead of an unhandled exception, with regression coverage for this case.

Written by the indexing model from the issue text.