Fixes <https://bugs.gnu.org/47007>.
Reported by Jelle Licht <jlicht@fsfe.org>.
Since commit dcb640f02b1f9590c3bd4301a22bf31bd60c56d4, we could end up
applying the wrong mount flags because the (find ...) expression could
pick the "wrong" mount point in the presence of bind mounts.
* gnu/build/file-systems.scm (mount-file-system): Use 'statfs' to
compute FLAGS whe FS is a bind mount.