40ignition-ostree: put growpart stamp file in /run

This stamp file was used to make sure coreos-growpart only ran on the
first boot when it ran in the real root. Nowadays, it runs in the
initramfs as part of `ignition-complete.target` before we even run
`ostree-prepare-root` (which meant we were actually creating the stamp
file in the initrd filesystem).

Having a stamp file is useful though for writing tests. So let's
repurpose the idea and put it in `/run` instead.

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-ostree/coreos-growpart

@@ -87,4 +87,5 @@ case "${TYPE}" in
     ext4) resize2fs "${src}" ;;
 esac
 
-touch /var/lib/coreos-growpart.stamp
+# this is useful for tests
+touch /run/coreos-growpart.stamp