title: How buildEnv builtin function works?
body: <p>How <a href="https://github.com/NixOS/nix/blob/master/corepkgs/buildenv.nix" rel="noreferrer">buildEnv</a> works? Why its builtin? How can I use it? <a href="https://github.com/NixOS/nix/blob/master/corepkgs/buildenv.nix#L1" rel="noreferrer">What does <code>manifest</code> argument?</a> Where is the documentation about buildEnv?</p>