Filter Nix-specific ANSI escape sequences from stderr

The Nixpkgs stdenv prints some custom escape sequences to denote
nesting and stuff like that. Most terminals (e.g. xterm, konsole)
ignore them, but some do not (e.g. xfce4-terminal). So for the benefit
of the latter, filter them out.
Reduce test verbosity
nix-store -l: Automatically pipe output into $PAGER