fix: initialize constraints list for install action

[JCapul]

Hi,
I hit a bug in the currently deployed version of JupyterLite (the try out version) when trying to use piplite install with a constraint file. I had a look at the code and realized the install action kwargs was not initialized for the "constraints" entry.
So here is a very quick fix.

I didn't find any related tests to update as well, so I thought I'd just issue this PR.

Let me know if I should update any other part to make it a proper fix.

To reproduce: %pip install -c pip_list.txt pep8

Then I got this error:

Traceback (most recent call last):
  File "/lib/python3.13/site-packages/pyodide_kernel/kernel.py", line 104, in run
    code = await self.lite_transform_manager.transform_cell(code)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/lib/python3.13/site-packages/pyodide_kernel/litetransform.py", line 32, in transform_cell
    lines = await transform(lines)
            ^^^^^^^^^^^^^^^^^^^^^^
  File "/lib/python3.13/site-packages/pyodide_kernel/litetransform.py", line 89, in pip_magic
    transformed_code = await piplite.cli.get_transformed_code(
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        shlex.split(pip_match[2])
        ^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/lib/python3.13/site-packages/piplite/cli.py", line 116, in get_transformed_code
    action, kwargs = await get_action_kwargs(argv)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/lib/python3.13/site-packages/piplite/cli.py", line 166, in get_action_kwargs
    kwargs["constraints"] += [spec]
    ~~~~~~^^^^^^^^^^^^^^^
KeyError: 'constraints'

[github-actions]

👈 Try it on ReadTheDocs

[bollwyvl]

LGTM

[bollwyvl]

this is fine, or could be a kwargs.setdefault("constraints", []).append() below.

[JCapul]

thx for the review, I find it simpler and more explicit as it is, but let me know if you prefer it the other way, no pb.