Python guest: re-fetch Python deps if `requirements.txt` changes

[crepererum]

What

If requirements.txt changes, we should re-fetch the Python dependencies.

Why

If one adds or updates dependencies, developers should get the newest version too. This is NOT an issue in CI since we don't cache that path. This is especially relevant once #104 is implemented.

How

We deliberately don't re-call pip every time we build the guest, since pip needs network IO and is generally not the fastest:

datafusion-udf-wasm/guests/python/Justfile

Lines 128 to 134 in 9e3d58a

	# skip if already downloaded
	if [ -d "{{PYTHON_TMP_VENV}}" ]; then
	echo "site-packages already set up"
	set +x
	echo ::endgroup::
	exit 0
	fi

However we should probably write a fingerprint file or something and check if that fingerprint is out-of-date.