systemd: ensure fsck Requires/After links are created in mount units

systemd-fsck-generator only produces these lines if it can find the
necessary fsck executable in its PATH.

fixes #29139.

nixos/modules/system/boot/stage-2-init.sh

@@ -159,6 +159,6 @@ exec {logOutFd}>&- {logErrFd}>&-
 
 # Start systemd.
 echo "starting systemd..."
-PATH=/run/current-system/systemd/lib/systemd \
+PATH=/run/current-system/systemd/lib/systemd:@fsPackagesPath@ \
     LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive \
     exec systemd

nixos/modules/system/boot/stage-2.nix

@@ -17,6 +17,7 @@ let
       pkgs.utillinux
       pkgs.openresolv
     ];
+    fsPackagesPath = lib.makeBinPath config.system.fsPackages;
     postBootCommands = pkgs.writeText "local-cmds"
       ''
         ${config.boot.postBootCommands}