prepare-root: Fix composefs + ostree admin unlock --hotfix compat

prepare-root: Fix composefs + ostree admin unlock --hotfix compat

There's a test case for `ostree admin unlock --hotfix` that
runs in FCOS, not here; it breaks when enabling composefs.

The reason is because the composefs is mounted readonly, and
we tried to remount it writable.  Instead of trying to remount
the rootfs writable at this point forcibly, honor the
*real* sysroot readonly state flag from the underlying FS before
we mounted the composefs.

Note that in FCOS derivatives we always have the root mounted
writable via `rw` on the kernel cmdline and this is the default
general expectation now with ostree usage.