swapspace: add passthru.binlore

Consider this Nix expression:

  let
    pkgs = import /path/to/nixpkgs { };
  in

  pkgs.resholve.writeScript "mime-construct-binlore-test" {
    inputs = [ pkgs.swapspace ];
    interpreter = "${pkgs.lib.getExe pkgs.bash}";
  } ''
    swapspace --version
  ''

Before this change, that expression would fail to build because resholve
would think that swapspace might be able to execute its arguments. This
commit fixes that problem.

pkgs/tools/admin/swapspace/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, autoreconfHook, installShellFiles, util-linux }:
+{ lib, stdenv, fetchFromGitHub, autoreconfHook, installShellFiles, util-linux, binlore, swapspace }:
 
 stdenv.mkDerivation rec {
   pname = "swapspace";
@@ -35,6 +35,12 @@ stdenv.mkDerivation rec {
     install --mode=444 -D 'swapspace.service' "$out/etc/systemd/system/swapspace.service"
   '';
 
+  # Nothing in swapspace --help or swapspace’s man page mentions
+  # anything about swapspace executing its arguments.
+  passthru.binlore.out = binlore.synthesize swapspace ''
+    execer cannot bin/swapspace
+  '';
+
   meta = with lib; {
     description = "Dynamic swap manager for Linux";
     homepage = "https://github.com/Tookmund/Swapspace";