ci: twister: Set twister timeout multiplier to 2

This commit sets the twister timeout multiplier to 2, which effectively
increases the default test timeout from 60 to 120 seconds, because the new
cost-effective Zephyr runners may take longer to execute tests and the
default timeout is not sufficient for some tests to complete.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>