nixos/systemd-initrd: create the /tmp mount point in the initrd

systemd now requires the /tmp mount point in the initrd cpio archive
since systemd/systemd#25723

setting `"/tmp/.keep".text` will create the directory.

this fixes a boot failure:
```
(sd-gens): Failed to overmount /tmp/: No such file or directory
```

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

@@ -371,6 +371,7 @@ in {
       managerEnvironment.PATH = "/bin:/sbin";
 
       contents = {
+        "/tmp/.keep".text = "systemd requires the /tmp mount point in the initrd cpio archive";
         "/init".source = "${cfg.package}/lib/systemd/systemd";
         "/etc/systemd/system".source = stage1Units;