fly: use completion generator

There's no need to ship the pre-generated completions in the nixpkgs
tree anymore.

pkgs/development/tools/continuous-integration/fly/default.nix

@@ -1,4 +1,4 @@
-{ buildGoModule, fetchFromGitHub, lib, writeText }:
+{ buildGoModule, fetchFromGitHub, stdenv, lib, writeText }:
 
 buildGoModule rec {
   pname = "fly";
@@ -20,12 +20,10 @@ buildGoModule rec {
       -X github.com/concourse/concourse.Version=${version}
   '';
 
-  # The fly.bash file included with this derivation can be replaced by a
-  # call to `fly completion bash` once the `completion` subcommand has
-  # made it into a release. Similarly, `fly completion zsh` will provide
-  # zsh completions. https://github.com/concourse/concourse/pull/4012
-  postInstall = ''
-    install -D -m 444 ${./fly.bash} $out/share/bash-completion/completions/fly
+  postInstall = lib.optionalString (stdenv.hostPlatform == stdenv.buildPlatform) ''
+    mkdir -p $out/share/{bash-completion/completions,zsh/site-functions}
+    $out/bin/fly completion --shell bash > $out/share/bash-completion/completions/fly
+    $out/bin/fly completion --shell zsh > $out/share/zsh/site-functions/_fly
   '';
 
   meta = with lib; {