curl: Ensure 'CURL_CA_BUNDLE' from host env is indeed respected

curl: Ensure 'CURL_CA_BUNDLE' from host env is indeed respected

Due to what looks like a copy'n'paste mistake, the environment setup script
might override 'CURL_CA_BUNDLE' from the host env instead of leaving it
untouched. Fix that.

(cherry picked from commit 545e43a7a45be02fda8fc3af69faa20e889f58c4)

CC: changqing.li@windriver.com
CC: raj.khem@gmail.com
CC: Peter.Marko@siemens.com
Signed-off-by: Moritz Haase <Moritz.Haase@bmw.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>