nixos: Fix create-needed-for-boot-dirs label-adressed dependencies

When a file system entry is specified by its label, not by its device,
we have to generate the device path from the label. While this was
already done in the script, it wasn't for the systemd unit
dependencies.

nixos.nix

@@ -695,10 +695,11 @@ in
       let
         neededForBootFs = catAttrs "mountPoint" (filter fsNeededForBoot (attrValues config.fileSystems));
         neededForBootDirs = filter (dir: elem dir.dirPath neededForBootFs) directories;
+        getDevice = fs: if fs.device != null then fs.device else "/dev/disk/by-label/${fs.label}";
         mkMount = fs:
           let
             mountPoint = concatPaths [ "/persist-tmp-mnt" fs.mountPoint ];
-            device = if fs.device != null then fs.device else "/dev/disk/by-label/${fs.label}";
+            device = getDevice fs;
             options = filter (o: (builtins.match "(x-.*\.mount)" o) == null) fs.options;
             optionsFlag = optionalString (options != [ ]) ("-o " + escapeShellArg (concatStringsSep "," options));
           in
@@ -725,7 +726,7 @@ in
               if fs.fsType == "zfs" then
                 "zfs-import.target"
               else
-                "${(escapeSystemdPath fs.device)}.device")
+                "${(escapeSystemdPath (getDevice fs))}.device")
             fileSystems);
         createNeededForBootDirs = ''
           ${concatMapStrings mkMount fileSystems}