composefs: Use lowerdir in /run

composefs: Use lowerdir in /run

I just noticed that this was another constant string duplicated
between prepare-root.c and libostree-1.so, and I went to make
it a common `#define` in libotcore.la.

But then I thought "it's ugly to have this directory mixed into
the deployment namespace" because in some theoretical world
it could also be in the ostree commit, which would cause weird
behavior.

I think this is transient state that is better in `/run`, so move
it there.