Clarify which environment gets activated first

[jaimergp]

Description

Docs were wrong / out of date for this one. Comes from conda-forge/conda-forge.github.io#1841 (comment)

cc @jakirkham

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

Proof:

conda-build/conda_build/build.py

Lines 2195 to 2238 in 8358206

	def _write_sh_activation_text(file_handle, m):
	cygpath_prefix = "$(cygpath -u " if utils.on_win else ""
	cygpath_suffix = " )" if utils.on_win else ""
	py_flags = "-I -m" if os.environ.get("_CONDA_BUILD_ISOLATED_ACTIVATION") else "-m"
	file_handle.write(
	f"""eval "$('{sys.executable}' {py_flags} conda shell.bash hook)"\n"""
	)
	if m.is_cross:
	# HACK: we need both build and host envs "active" - i.e. on PATH,
	# and with their activate.d scripts sourced. Conda only
	# lets us activate one, though. This is a
	# vile hack to trick conda into "stacking"
	# two environments.
	#
	# Net effect: binaries come from host first, then build
	#
	# Conda 4.4 may break this by reworking the activate scripts.
	# ^^ shouldn't be true
	# In conda 4.4, export CONDA_MAX_SHLVL=2 to stack envs to two
	# levels deep.
	# conda 4.4 does require that a conda-meta/history file
	# exists to identify a valid conda environment
	# conda 4.6 changes this one final time, by adding a '--stack' flag to the 'activate'
	# command, and 'activate' does not stack environments by default without that flag
	history_file = join(m.config.host_prefix, "conda-meta", "history")
	if not isfile(history_file):
	if not isdir(dirname(history_file)):
	os.makedirs(dirname(history_file))
	open(history_file, "a").close()
	host_prefix_path = "".join(
	(cygpath_prefix, m.config.host_prefix.replace("\\", "\\\\"), cygpath_suffix)
	)
	file_handle.write(f'conda activate "{host_prefix_path}"\n')
	# Write build prefix activation AFTER host prefix, so that its executables come first
	build_prefix_path = "".join(
	(cygpath_prefix, m.config.build_prefix.replace("\\", "\\\\"), cygpath_suffix)
	)
	# Do not stack against base env when not cross.
	stack = "--stack" if m.is_cross else ""
	file_handle.write(f'conda activate {stack} "{build_prefix_path}"\n')