nixos-rebuild: add flake support for build-vm

This relies on users using `nixpkgs.lib.nixosSystem` to define their
system; otherwise, the `vm` and `vmWithBootLoader` attributes will not
exist.

(cherry picked from commit e26b348689)

flake.nix

@@ -5,7 +5,6 @@
 
   outputs = { self }:
     let
-
       jobs = import ./pkgs/top-level/release.nix {
         nixpkgs = self;
       };
@@ -28,10 +27,31 @@
       lib = lib.extend (final: prev: {
         nixosSystem = { modules, ... } @ args:
           import ./nixos/lib/eval-config.nix (args // {
-            modules = modules ++
-              [ { system.nixos.versionSuffix =
+            modules =
+              let
+                vmConfig = (import ./nixos/lib/eval-config.nix
+                  (args // {
+                    modules = modules ++ [ ./nixos/modules/virtualisation/qemu-vm.nix ];
+                  })).config;
+
+                vmWithBootLoaderConfig = (import ./nixos/lib/eval-config.nix
+                  (args // {
+                    modules = modules ++ [
+                      ./nixos/modules/virtualisation/qemu-vm.nix
+                      { virtualisation.useBootLoader = true; }
+                    ];
+                  })).config;
+              in
+              modules ++ [
+                {
+                  system.nixos.versionSuffix =
                     ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified)}.${self.shortRev or "dirty"}";
                   system.nixos.revision = final.mkIf (self ? rev) self.rev;
+
+                  system.build = {
+                    vm = vmConfig.system.build.vm;
+                    vmWithBootLoader = vmWithBootLoaderConfig.system.build.vm;
+                  };
                 }
               ];
           });

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -438,15 +438,17 @@ if [ -z "$rollback" ]; then
         if [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A vm -k "${extraBuildFlags[@]}")"
         else
-            echo "$0: 'build-vm' is not supported with '--flake'" >&2
-            exit 1
+            nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.vm" \
+              "${extraBuildFlags[@]}" "${lockFlags[@]}"
+            pathToConfig="$(readlink -f ./result)"
         fi
     elif [ "$action" = build-vm-with-bootloader ]; then
         if [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A vmWithBootLoader -k "${extraBuildFlags[@]}")"
         else
-            echo "$0: 'build-vm-with-bootloader' is not supported with '--flake'" >&2
-            exit 1
+            nix "${flakeFlags[@]}" build "$flake#$flakeAttr.config.system.build.vmWithBootLoader" \
+              "${extraBuildFlags[@]}" "${lockFlags[@]}"
+            pathToConfig="$(readlink -f ./result)"
         fi
     else
         showSyntax