ci: cache openwrt staging_dir outside openwrt directory

Previously, we've cached staging_dir in the gluon/openwrt subdirectory.
But this doesn't really work properly because of gitlab's submodule
behaviour. It will first extract the cache and then update the
submodules.

Because the cached directory resides inside the gluon submodule, it will
be deleted upon submodule checkout. By putting the cache in a directory
outside the submodule we can avoid this. We will then simply symlink the
cached directory to the desired destination such that it is found during
the build.