flake.nix: add prebuild-all-remote

flake.nix

@@ -115,6 +115,20 @@
             '') (builtins.attrNames flakifiedHosts)}
           '';
 
+          prebuild-all-remote = pkgs.writeScriptBin "prebuild-all" ''
+            #!${pkgs.runtimeShell} -e
+
+            nix copy --to ssh://$1 ${secrets}
+            nix copy --to ssh://$1 ${self}
+
+            set -x
+            ssh $1 -- nix build -vL --no-link ${
+              pkgs.lib.concatMapStringsSep " " (name:
+                "${self}#nixosConfigurations.${name}.config.system.build.toplevel"
+              ) (builtins.attrNames flakifiedHosts)
+            }
+          '';
+
         } //
 
         overlayPkgs //