nixos/systemd-stage-1: explicitly add pkgs.runtimeShell dependency

Because it's no longer pkgs.bash (which is interactive), and
can be something else on other platforms anyway.

nixos/modules/system/boot/systemd/initrd.nix

@@ -538,7 +538,9 @@ in
           "${cfg.package.util-linux}/bin/umount"
           "${cfg.package.util-linux}/bin/sulogin"
 
-          # required for script services, and some tools like xfs still want the sh symlink
+          # required for services generated with writeShellScript and friends
+          pkgs.runtimeShell
+          # some tools like xfs still want the sh symlink
           "${pkgs.bash}/bin"
 
           # so NSS can look up usernames