lab: generate and install zsh completion

Env variables are set to dummy values
so `lab` doesn't get stuck trying to helpfully
interact with user for initial configuration.

pkgs/applications/version-management/git-and-tools/lab/default.nix

@@ -17,6 +17,12 @@ buildGoPackage rec {
 
   goDeps = ./deps.nix;
 
+  postInstall = ''
+    mkdir -p $out/share/zsh/site-functions
+    LAB_CORE_HOST=a LAB_CORE_USER=b LAB_CORE_TOKEN=c \
+    $out/bin/lab completion zsh > $out/share/zsh/site-functions/_lab
+  '';
+
   meta = with stdenv.lib; {
     description = "Lab wraps Git or Hub, making it simple to clone, fork, and interact with repositories on GitLab";
     homepage = https://zaquestion.github.io/lab;