cmake: shields: escaping and quoting list when used for env settings

Fixes: #30553

The `;` in a list must be escaped when used as for execute_process and
add_custom_target, as those functions are using `cmake -E env` to setup
correct environment before executing kconfig and menuconfig.

This commits escapes `SHIELD_AS_LIST` variable list correctly before
using it.

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