clone.sh: Switch default dependency branch to "main"

TF-M repos are going to switch development branch from "master" to
"main". So update the default dependency branch in CI to "main".

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: If64a9e90bcbde092b2d6c5f0761f2fe9088d80bc
1 file changed