ci: codecov: Set twister timeout multiplier to 2

This commit sets the codecov workflow 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>