Use a fixed directory in /run/... as lower dir

[travier]

Until we start using and relying on loop less mounting / direct file/fd mounting, we should use a fixed directory in /run/... as lower dir instead of a temporary one in /tmp.

[root@fedora ~]# findmnt /
TARGET SOURCE FSTYPE  OPTIONS
/      none   overlay ro,relatime,seclabel,lowerdir+=/tmp/.tmpvKg9Y9,datadir+=/sysroot/composefs/objects,redirect_dir=on,metacopy=on,verity=require

composefs/ostree:

coreos@fcos$ findmnt /
TARGET SOURCE    FSTYPE  OPTIONS
/      composefs overlay ro,relatime,seclabel,lowerdir+=/run/ostree/.private/cfsroot-lower,datadir+=/sysroot/ostree/repo/objects,redirect_dir=on,metacopy=on

[allisonkarlitskaya]

More likely to go with #64 on this one...