This is an alternative to the IN_NIX_SHELL environment variable,
allowing the expression to adapt itself to nix-shell without
triggering those adaptations when used as a dependency of another
shell.
Closes#3147
The nix-shell fix in 668fef2e4f revealed
that we had some --pure tests that incorrectly depended on PATH from
config.nix's mkDerivation being overwritten by the caller's PATH.
http://hydra.nixos.org/build/49242478