package/pkg-cmake.mk: use /run as runtime path

Since many years now, /run is the standard runstate directory for Linux
systems, and Buildroot follows this standard. Encode this as the default
for CMake packages.

Note that we don't really care for host packages.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>