Respect env variables in all build environments

[stsewd]

I guess is to get around any security implications.
But modifying the PATH to a local copy of the environments variables of
the host leads to errors.

This was ok when using the local host to run the checkout steps, but now we are running inside the container.

Also, we already set all the READTHEDOCS env variables from

readthedocs.org/readthedocs/projects/tasks.py

Lines 725 to 762 in 7212d6f

	def get_env_vars(self):
	"""Get bash environment variables used for all builder commands."""
	env = {
	'READTHEDOCS': True,
	'READTHEDOCS_VERSION': self.version.slug,
	'READTHEDOCS_PROJECT': self.project.slug,
	'READTHEDOCS_LANGUAGE': self.project.language,
	}
	if self.config.conda is not None:
	env.update({
	'CONDA_ENVS_PATH': os.path.join(self.project.doc_path, 'conda'),
	'CONDA_DEFAULT_ENV': self.version.slug,
	'BIN_PATH': os.path.join(
	self.project.doc_path,
	'conda',
	self.version.slug,
	'bin',
	),
	})
	else:
	env.update({
	'BIN_PATH': os.path.join(
	self.project.doc_path,
	'envs',
	self.version.slug,
	'bin',
	),
	})
	# Update environment from Project's specific environment variables,
	# avoiding to expose environment variables if the version is external
	# for security reasons.
	if self.version.type != EXTERNAL:
	env.update(self.project.environment_variables)
	return env

This is needed for #6436

[stsewd]

Also, in production the local environment was only used in the checkout step.

[stsewd]

I split this PR into 3 to see have more idea where the incompatible change is happening.

#6480
#6481

Thos PRs can be merged and it won't break current installations or .com