boards: st: stm32n6570_dk: exclude stm32n6_fsbl variant from twister ci

Make the stm32n6_dk variant (stm32n6570_dk/stm32n657xx/fsbl) unable
to execute tests in CI with Twister.

This change will help avoid many CI build failures caused by this variant.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>