nixos: Fix fsck with systemd 251.6 and later
nixos: Fix fsck with systemd 251.6 and later

Version 251.6 of systemd introduced a small change[1] that now checks
whether the fsck command is available in *addition* to the filesystem
specific fsck.$fsname executable.

When bumping systemd to version 251.7 on our side[2], we introduced that
change. This subsequently caused our "fsck" test to fail and it looks
like this was an oversight during the pull request[3] introducing the
bump.

Since the fsck wrapper binary is in util-linux, I decided to address
this by adding util-linux to fsPackages because util-linux is already
part of the closure of any NixOS system so the impact should be pretty
low.

[1]: https://github.com/systemd/systemd-stable/commit/73db7d99323c236625656f906eb4e429613d324b
[2]: https://github.com/NixOS/nixpkgs/commit/844a08cc06b5c0703ba37f2318ef5b7d90665d04
[3]: https://github.com/NixOS/nixpkgs/pull/199618

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@nix.build>

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