nixos/rpcbind: add dependency for systemd-tmpfiles-setup

The inline comments explain the reasoning behind this change. This
work was initiated due to failing tests explicitly for glusterfs, but
my hunch is that any nixosTest adjacent to rpcbind will start working
again.

Ref: https://github.com/NixOS/nixpkgs/issues/175339

nixos/modules/services/networking/rpcbind.nix

@@ -35,6 +35,16 @@ with lib;
 
     systemd.services.rpcbind = {
       wantedBy = [ "multi-user.target" ];
+      # rpcbind performs a check for /var/run/rpcbind.lock at startup
+      # and will crash if /var/run isn't present. In the stock NixOS
+      # var.conf tmpfiles configuration file, /var/run is symlinked to
+      # /run, so rpcbind can enter a race condition in which /var/run
+      # isn't symlinked yet but tries to interact with the path, so
+      # controlling the order explicitly here ensures that rpcbind can
+      # start successfully. The `wants` instead of `requires` should
+      # avoid creating a strict/brittle dependency.
+      wants = [ "systemd-tmpfiles-setup.service" ];
+      after = [ "systemd-tmpfiles-setup.service" ];
     };
 
     users.users.rpc = {