refactor(host/orion): improve fs layout

hosts/orion/os/fs.nix

@@ -1,4 +1,7 @@
-{ lib, ... }:
+{
+  lib ? (import <nixpkgs> { }).lib,
+  ...
+}:
 let
   root-disk = "/dev/nvme0n1";
   persist-dir = "/persist";
@@ -38,18 +41,13 @@ in
       content = {
         type = "gpt";
         partitions = {
-          esp =
-            let
-              label = "NixOS-Boot";
-            in
-            {
+          esp = {
             priority = 1;
             size = "512M";
             type = "EF00";
             content = {
               extraArgs = [
-                  "-n ${label}"
-                  "-F 32"
+                "-n 'NixOS-Boot'"
               ];
               type = "filesystem";
               format = "vfat";
@@ -60,11 +58,7 @@ in
               ];
             };
           };
-          root =
-            let
-              label = "NixOS-Primary";
-            in
-            {
+          root = {
             size = "100%";
             content = {
               type = "luks";
@@ -77,13 +71,10 @@ in
                 type = "btrfs";
                 extraArgs = [
                   "-f"
-                    "--label ${label}"
+                  "-L NixOS-Primary"
                 ];
-                  postCreateHook = ''
-                    MOUNT="$(mktemp -d)"
-                    mount "/dev/disk/by-label/${label}" "$MOUNT" -o subvol=/
-                    trap 'umount $MOUNT; rm -rf $MOUNT' EXIT
-                    btrfs subvolume snapshot -r "$MOUNT/root" "$MOUNT/root-base"
+                preUnmountHook = ''
+                  btrfs subvolume snapshot -r "root" "root-base"
                 '';
                 subvolumes = {
                   "/root" = {