prometheus-bird-exporter: update birdSocket default path

a5276e1fbd updated bird to use
/run/bird/bird.ctl, but didn't update the default used for
prometheus-bird-exporter.

Reported-In: https://github.com/NixOS/nixpkgs/pull/161193#issuecomment-1086228985

nixos/modules/services/monitoring/prometheus/exporters/bird.nix

@@ -17,7 +17,7 @@ in
     };
     birdSocket = mkOption {
       type = types.path;
-      default = "/var/run/bird.ctl";
+      default = "/run/bird/bird.ctl";
       description = ''
         Path to BIRD2 (or BIRD1 v4) socket.
       '';