Address remaining PR review comments

- Pin cache-apt-pkgs-action to commit SHA for supply-chain safety
- Fix Homebrew post_install to use with_env block instead of env hash
  in system() call (idiomatic Homebrew pattern)
- Add clarifying comments to service file, preremove.sh, and nfpm.yaml
  explaining user/group creation, directory ownership, and upgrade handling

https://claude.ai/code/session_01Vx1EsNrNySgsc8Y67dGzCn

brew_dist/archivebox.rb

@@ -41,7 +41,9 @@ class Archivebox < Formula
   def post_install
     # Install runtime dependencies (plugins, JS extractors, etc.)
     (var/"archivebox").mkpath
-    system({ "DATA_DIR" => var/"archivebox" }, bin/"archivebox", "install", "--binproviders", "pip,npm")
+    with_env(DATA_DIR: var/"archivebox") do
+      system bin/"archivebox", "install", "--binproviders", "pip,npm"
+    end
   end
 
   service do