In [upstream change][1], handling of creating `/run/mdadm/creating-%s`
file was changed to fail if it was unable to create the file. This
revealed that the path itself incorrectly hardcodes `/run/mdadm` instead
of using `MAP_DIR` or similar. Since nixpkgs sets `RUN_DIR=/run/.mdadm`
at compile time, and `MAP_DIR=$(RUN_DIR)` in upstream, the `/run/mdadm`
is never created. This remedies by fixing the hard-coded directory.
[1]: https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=9f376da6439b07dc93ae084ab576e133b9d8d839
