nixos/systemd: re-add openssh to manager PATH

Otherwise systemd-ssh-generator doesn't work.

This can only be removed once
0013-inherit-systemd-environment-when-calling-generators.patch is
dropped.

nixos/modules/system/boot/systemd.nix

@@ -713,7 +713,12 @@ in
     systemd.managerEnvironment = {
       # Doesn't contain systemd itself - everything works so it seems to use the compiled-in value for its tools
       # util-linux is needed for the main fsck utility wrapping the fs-specific ones
-      PATH = lib.makeBinPath (config.system.fsPackages ++ [ cfg.package.util-linux ]);
+      PATH = lib.makeBinPath (
+        config.system.fsPackages
+        ++ [ cfg.package.util-linux ]
+        # systemd-ssh-generator needs sshd in PATH
+        ++ lib.optional config.services.openssh.enable config.services.openssh.package
+      );
       LOCALE_ARCHIVE = "/run/current-system/sw/lib/locale/locale-archive";
       TZDIR = "/etc/zoneinfo";
       # If SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable