shield: cmake: adjust if(EXISTS <shield>.conf) to check for right file

Fixes: #32269

This commit is a followup to #31877 where the `${shield_dir}/${s_dir}`
was adjusted to `${SHIELD_DIR_${s}}` to ensure only including shields
and not boards in list of known shields.

An `if(EXISTS ${shield_dir}/${s_dir}...)` was missed in this process
which causes #32269. This is corrected in this commit.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>