nixos/tailscale: add procps to $PATH

Currently tailscaled expects `sysctl` (from package procps) to be present
in the path when running on Linux. It can function without the `sysctl`
command present but it prints an error about it. This fixes that error.

    Warning: couldn't check net.ipv4.ip_forward (exec: "sysctl":
        executable file not found in $PATH).

Signed-off-by: Christine Dodrill <me@christine.website>

nixos/modules/services/networking/tailscale.nix

@@ -28,7 +28,7 @@ in {
     systemd.packages = [ cfg.package ];
     systemd.services.tailscaled = {
       wantedBy = [ "multi-user.target" ];
-      path = [ pkgs.openresolv ];
+      path = [ pkgs.openresolv pkgs.procps ];
       serviceConfig.Environment = "PORT=${toString cfg.port}";
     };
   };