Create a separate file for declaring vm fixes

lib/config/custom-build-vm.nix

@@ -0,0 +1,35 @@
+{ config, lib, pkgs, ... }: {
+  config = {
+    # Replace native `qemu` with `remote-viewer`
+    system.build = {
+      vmWithBootLoader = pkgs.writeShellApplication {
+        name = "run-${config.system.name}-vm";
+        text = ''
+          ${config.virtualisation.vmVariant.system.build.vm}/bin/run-${config.system.name}-vm
+        '';
+      };
+    };
+
+    virtualisation =
+      let config = {
+        boot.loader.efi.efiSysMountPoint = lib.mkForce "/boot";
+
+        virtualisation = {
+          qemu.options = [
+            "-display sdl"
+          ];
+
+          sharedDirectories = {
+            hostKeys = {
+              source = "/etc/ssh";
+              target = "/etc/ssh";
+            };
+          };
+        };
+      };
+      in {
+        vmVariant = config;
+        vmVariantWithBootLoader = config;
+      };
+  };
+}

lib/system.nix

@@ -17,29 +17,9 @@
                             { config, pkgs, lib, ... }: {
                                 imports = [
                                     machineConfig
+                                    ./config/custom-build-vm.nix
                                 ];
 
-                                virtualisation =
-                                    let
-                                        config = {
-                                            boot.loader.efi.efiSysMountPoint = lib.mkVMOverride "/boot";
-
-                                            virtualisation.qemu.options = [
-                                                "-display sdl"
-                                            ];
-
-                                            virtualisation.sharedDirectories = {
-                                                keys = {
-                                                    source = "/etc/ssh";
-                                                    target = "/etc/ssh";
-                                                };
-                                            };
-                                        };
-                                    in {
-                                        vmVariant = config;
-                                        vmVariantWithBootLoader = config;
-                                    };
-
                                 # Networking
                                 networking.hostName = name;
                             })