Avoids the call to `os.Getenv` when it is unnecessary because it would be overwritten anyway. Removes the comments about moving environment variable parsing to a method for use by library consumers. That method exists in containerd and this set of code doesn't actually perform any parsing since the parsing of this time is done within buildkit and not on the client. Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>