handlr: install shell completions for zsh and fish

pkgs/tools/misc/handlr/default.nix

···
-{ lib, stdenv, rustPlatform, fetchFromGitHub, shared-mime-info, libiconv }:
+{ lib, stdenv, rustPlatform, fetchFromGitHub, shared-mime-info, libiconv, installShellFiles }:
 
 rustPlatform.buildRustPackage rec {
   pname = "handlr";
···
 
   cargoSha256 = "sha256-xDQV8wVlzItz0lzR1nVRPVsg7nSf/khUhevDlGgSO3g=";
 
-  nativeBuildInputs = [ shared-mime-info ];
+  nativeBuildInputs = [ installShellFiles shared-mime-info ];
   buildInputs = lib.optional stdenv.isDarwin libiconv;
 
   preCheck = ''
     export HOME=$TEMPDIR
+  '';
+
+  postInstall = ''
+    installShellCompletion \
+      --zsh  completions/_handlr \
+      --fish completions/handlr.fish
   '';
 
   meta = with lib; {