app2unit: fix missing runtime dependency

The runtime dependency of xdg-terminal-exec is missing from app2unit.
This commit replaces `xdg-terminal-exec` with the nix store path of the xdg-terminal-exec executable. This makes it possible to run terminal applications with app2unit without having to install xdg-terminal-exec as well.

pkgs/by-name/ap/app2unit/package.nix

@@ -2,6 +2,7 @@
   lib,
   stdenvNoCC,
   dash,
+  xdg-terminal-exec,
   scdoc,
   fetchFromGitHub,
   nix-update-script,
@@ -48,7 +49,9 @@ stdenvNoCC.mkDerivation (finalAttrs: {
   dontPatchShebangs = true;
   postFixup = ''
     substituteInPlace $out/bin/app2unit \
-      --replace-fail '#!/bin/sh' '#!${lib.getExe dash}'
+      --replace-fail '#!/bin/sh' '#!${lib.getExe dash}' \
+      --replace-fail 'A2U__TERMINAL_HANDLER=xdg-terminal-exec' \
+                     'A2U__TERMINAL_HANDLER=${lib.getExe xdg-terminal-exec}'
   '';
 
   meta = {