nixos/systemd-stage-1: follow systemd /run propagation

We currently bypass systemd's switch-root logic by premounting
/sysroot/run. Make sure to propagate its sub-mounts with the recursive
flag, in accordance with the default switch-root logic.

This is required for creds at /run/credentials to survive the transition
from initrd -> host.

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

@@ -642,7 +642,7 @@ in
         {
           where = "/sysroot/run";
           what = "/run";
-          options = "bind";
+          options = "rbind";
           unitConfig = {
             # See the comment on the mount unit for /run/etc-metadata
             DefaultDependencies = false;