Drop `script_env` workaround for `PIP_*` variables in Python build

[jakirkham]

Previously to workaround issues with PIP_* environment variables not being set in multiple output recipes, this recipe set them manually

xgboost-feedstock/recipe/meta.yaml

Lines 108 to 115 in 21bfb2f

	script_env:
	# Workaround an upstream conda-build issue w/pip & `outputs` by setting env vars manually.
	# xref: https://github.com/conda/conda-build/issues/3993
	- PIP_NO_BUILD_ISOLATION=False
	- PIP_NO_DEPENDENCIES=True
	- PIP_IGNORE_INSTALLED=True
	- PIP_CACHE_DIR=pip_cache
	- PIP_NO_INDEX=True

Since this workaround was added, conda-build 24.7 include a fix for this issue: conda/conda-build#5368

As a result, these should no longer be needed. So this block can be dropped