Makefile: apply PYTHON_ARG override in all uv commands

[kevinjqliu]

Rationale for this change

Follow up to #2601

Apply PYTHON_ARG override to all uv commands`

Are these changes tested?

Yes

Override to use python 3.13

export PYTHON=3.13
make setup-venv
make install
make docs-serve

Unset override and verify it uses system default, python 3.12

unset PYTHON
make setup-venv
make test

Are there any user-facing changes?

No

[kevinjqliu]

cc @geruh