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]: 73db7d9932
[2]: 844a08cc06
[3]: https://github.com/NixOS/nixpkgs/pull/199618

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

nixos/modules/tasks/filesystems.nix

@@ -300,7 +300,11 @@ in
     boot.supportedFilesystems = map (fs: fs.fsType) fileSystems;
 
     # Add the mount helpers to the system path so that `mount' can find them.
-    system.fsPackages = [ pkgs.dosfstools ];
+    system.fsPackages = [
+      pkgs.dosfstools
+      # This is needed for the main fsck utility wrapping the fs-specific ones.
+      pkgs.util-linux
+    ];
 
     environment.systemPackages = with pkgs; [ fuse3 fuse ] ++ config.system.fsPackages;