automation/gitlab: add https transport support to Debian images

automation/gitlab: add https transport support to Debian images

The LLVM repos have switched from http to https, and trying to access
using http will get redirected to https. Add the apt-transport-https
package to the x86 Debian containers that use the LLVM repos, in order
to support the https transport method.

Note that on Arm we only test with gcc, so don't add the package for
the Debian Arm container.

This fixes the following error seen on the QEMU smoke tests:

E: The method driver /usr/lib/apt/methods/https could not be found.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wl@xen.org>