Fix expandOnBoot=false and rigid path to nix-path-registration (#341071)

nixos/modules/installer/sd-card/sd-image.nix

@@ -150,6 +150,15 @@ in
         Whether to configure the sd image to expand it's partition on boot.
       '';
     };
+
+    nixPathRegistrationFile = mkOption {
+      type = types.str;
+      default = "/nix-path-registration";
+      description = ''
+        Location of the file containing the input for nix-store --load-db once the machine has booted.
+        If overriding fileSystems."/" then you should to set this to the root mount + /nix-path-registration
+      '';
+    };
   };
 
   config = {
@@ -255,11 +264,8 @@ in
       '';
     }) {};
 
-    boot.postBootCommands = lib.mkIf config.sdImage.expandOnBoot ''
-      # On the first boot do some maintenance tasks
-      if [ -f /nix-path-registration ]; then
-        set -euo pipefail
-        set -x
+    boot.postBootCommands = let
+      expandOnBoot = lib.optionalString config.sdImage.expandOnBoot ''
         # Figure out device names for the boot device and root filesystem.
         rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
         bootDevice=$(lsblk -npo PKNAME $rootPart)
@@ -269,16 +275,25 @@ in
         echo ",+," | sfdisk -N$partNum --no-reread $bootDevice
         ${pkgs.parted}/bin/partprobe
         ${pkgs.e2fsprogs}/bin/resize2fs $rootPart
+      '';
+      nixPathRegistrationFile = config.sdImage.nixPathRegistrationFile;
+    in ''
+      # On the first boot do some maintenance tasks
+      if [ -f ${nixPathRegistrationFile} ]; then
+        set -euo pipefail
+        set -x
+
+        ${expandOnBoot}
 
         # Register the contents of the initial Nix store
-        ${config.nix.package.out}/bin/nix-store --load-db < /nix-path-registration
+        ${config.nix.package.out}/bin/nix-store --load-db < ${nixPathRegistrationFile}
 
         # nixos-rebuild also requires a "system" profile and an /etc/NIXOS tag.
         touch /etc/NIXOS
         ${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/system --set /run/current-system
 
         # Prevents this from running on later boots.
-        rm -f /nix-path-registration
+        rm -f ${nixPathRegistrationFile}
       fi
     '';
   };