doc/stdenv: put note about remote builds with breakpointHook into a <note>
doc/stdenv: put note about remote builds with breakpointHook into a <note>

With remote builds, the sandbox can't be accessed by `cntr` as it is on
a different machine. I decided to put this into an extra `note` block as it took
me admittedly too much time to figure this out.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs