buildtools-tarball: export certificates to python and curl

The custom path of the ca-certificates.crt within the buildtools-tarball requires more
environment variables to be exported. Namely REQUESTS_CA_BUNDLE for the python requests library
and CURL_CA_BUNDLE for curl.

Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>