Move let bindings to the top

lib/config/custom-build-vm.nix

@@ -1,4 +1,53 @@
-{ config, lib, options, pkgs, ... }: {
+{ config, lib, options, pkgs, ... }:
+let
+  packageName = "custom-nixos-vm";
+
+  mergedSystemOption =
+    with options.system;
+    lib.mergeDefinitions
+      build.loc
+      build.type
+      (lib.lists.forEach
+        (
+          builtins.filter
+            (item:
+              !(lib.path.hasPrefix ./. (/. + item.file)))
+            build.definitionsWithLocations)
+        (item: { inherit (item) file value; }));
+
+  vanillaVM = mergedSystemOption.mergedValue.vm;
+
+  createVMWrapper =
+    vm:
+      if (vm.name == packageName)
+      then
+        vm
+      else
+        let
+          prefix =
+            lib.concatStringsSep " " (
+              lib.optionals config.virtualisation.runAsRoot (
+                ["sudo"] ++
+                lib.optional config.virtualisation.qemu.runInBackground "-b"));
+
+          suffix =
+            lib.concatStringsSep " " (
+              lib.optional
+                (!config.virtualisation.runAsRoot && config.virtualisation.qemu.runInBackground)
+                "&");
+
+          wrapped = pkgs.writeShellApplication {
+            name = "run-${config.system.name}-vm";
+            text = ''
+              ${prefix} ${vm}/bin/run-${config.system.name}-vm ${suffix}
+            '';
+          };
+        in
+          pkgs.symlinkJoin {
+            name = packageName;
+            paths = [ wrapped ];
+          };
+in {
   options =
   let
     vmVariantOptions = {
@@ -57,54 +106,7 @@
     # Replace native `qemu` with `remote-viewer`
     system.build =
       {
-        vm =
+        vm = lib.mkForce (createVMWrapper vanillaVM);
-          let
-            packageName = "custom-nixos-vm";
-
-            mergedSystem =
-              with options.system;
-              lib.mergeDefinitions
-                build.loc
-                build.type
-                (lib.lists.forEach
-                  (
-                    builtins.filter
-                      (item:
-                        !(lib.path.hasPrefix ./. (/. + item.file)))
-                      build.definitionsWithLocations)
-                  (item: { inherit (item) file value; }));
-          in
-            lib.mkForce (
-              with { inherit (mergedSystem.mergedValue) vm; };
-                if (vm.name == packageName)
-                then
-                  vm
-                else
-                  let
-                    prefix =
-                      lib.concatStringsSep " " (
-                        lib.optionals config.virtualisation.runAsRoot (
-                          ["sudo"] ++
-                          lib.optional config.virtualisation.qemu.runInBackground "-b"));
-
-                    suffix =
-                      lib.concatStringsSep " " (
-                        lib.optional
-                          (!config.virtualisation.runAsRoot && config.virtualisation.qemu.runInBackground)
-                          "&");
-
-                    wrapped = pkgs.writeShellApplication {
-                      name = "run-${config.system.name}-vm";
-                      text = ''
-                        ${prefix} ${vm}/bin/run-${config.system.name}-vm ${suffix}
-                      '';
-                    };
-                  in
-                    # Rename package to `nixos-vm`
-                    pkgs.symlinkJoin {
-                      name = packageName;
-                      paths = [ wrapped ];
-                    });
       };
   };
 }