cmake: Fix cache directory detection

To detect where the cache directory has been located we have been
checking if $HOME is writable, and if it is assuming that $HOME/.cache
must be writable as well.

This is broken in environments where $HOME is owned by users and
$HOME/.cache is owned by admin. To fix this we check $HOME/.cache for
write-ability instead.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>