helpers: mount: fix rootfs mount path (MR 2344)

Subtle...

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/helpers/mount.py

@@ -109,6 +109,6 @@ def mount_device_rootfs(chroot_rootfs: Chroot) -> PurePath:
                           "rootfs_qemu-amd64")
     :returns: the mountpoint (relative to the native chroot)
     """
-    mountpoint = PurePath("/mnt", chroot_rootfs.dirname)
+    mountpoint = PurePath("/mnt", str(chroot_rootfs))
     pmb.helpers.mount.bind(chroot_rootfs.path, Chroot.native() / mountpoint)
     return mountpoint