Palace is not available when generating docs in CI

[nikosavola]

Describe the bug
For example in https://github.com/gdsfactory/gplugins/actions/runs/17824958480/job/50675881265, we get:

File ~/work/gplugins/gplugins/gplugins/palace/get_capacitance.py:540, in run_capacitive_simulation_palace(component, element_order, n_processes, layer_stack, material_spec, simulation_folder, simulator_params, mesh_parameters, mesh_file)
    525 gmsh.finalize()
    527 _generate_json(
    528     simulation_folder,
    529     component.name,
   (...)    538     simulator_params,
    539 )
--> 540 _palace(simulation_folder,filename,n_processes)
    542 results = _read_palace_results(
    543     simulation_folder,
    544     filename,
    545     [port.name for port in component.ports],
    546     is_temporary=str(simulation_folder) == temp_dir.name,
    547 )
    548 temp_dir.cleanup()

File ~/work/gplugins/gplugins/gplugins/palace/get_capacitance.py:199, in _palace(simulation_folder, name, n_processes)
    197     except Exception as e:
    198         print(f"   ❌ Failed to run Palace via Spack: {e}")
--> 199         raise RuntimeError(
    200             "palace not found. Make sure it is available in your PATH or via Spack."
    201         )
    202 else:
    203     # Palace found in PATH, use the original method
    204     json_file = simulation_folder / f"{Path(name).stem}.json"

RuntimeError: palace not found. Make sure it is available in your PATH or via Spack.

To Reproduce
Run the Sphinx docs to gh-pages flow

Expected behavior
Notebooks using Palace run

Tasks

• Have Palace available from an apptainer container when building the docs similarly to test_code.yml -> test_palace job.
• Reuse the same cache for palace.sif generated with apptainer build from the test_palace job if possible.