docker,worker: install python3-cryptography

docker,worker: install python3-cryptography

Addition of OPTEE support along with STM32 added a requirment on
python3-cryptography and without it buildbots for STM32 will fail with:
Checking 'python3-cryptography'... failed.
Checking 'python3-pyelftools'... ok.
optee-os: Please install the Python3 cryptography module
make[3]: *** [/builder/shared-workdir/build/include/prereq.mk:9: prereq] Error 1

So install python3-cryptography to satisfy it.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Link: https://github.com/openwrt/buildbot/pull/61
Signed-off-by: Petr Štetiar <ynezz@true.cz>