sysbuild: cmake: fix sysbuild_cache_set() function

The sysbuild_cache_set() supports setting variables in a dedicated
sysbuild image cache. When appending to a list, the list is checked for
existance of the variable to avoid appending it again on reruns.

This was done be checking with `string(FIND ...)`, but that will also
return an index on a partial match.

A much safer approach is to use the `if(<val> IN_LIST <list>)` approach
to check for existance.

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