Disable automatic submodule handling

As of https://github.com/Mbed-TLS/mbedtls-test/pull/206 the CI scripts
handle submodules manually. Disable automatic handling which would
interfere with the proper functioning of restricted PR jobs.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: Id40c63e363aa35a08d6f2da9813e91cff5492978
7 files changed