aria2: fix completions install

pkgs/tools/networking/aria2/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchpatch, fetchFromGitHub, pkg-config, autoreconfHook
+{ lib, stdenv, fetchFromGitHub, pkg-config, autoreconfHook
 , openssl, c-ares, libxml2, sqlite, zlib, libssh2
 , cppunit, sphinx
 , Security
@@ -26,6 +26,7 @@ stdenv.mkDerivation rec {
   configureFlags = [
     "--with-ca-bundle=/etc/ssl/certs/ca-certificates.crt"
     "--enable-libaria2"
+    "--with-bashcompletiondir=${placeholder "bin"}/share/bash-completion/completions"
   ];
 
   prePatch = ''