This allows shell.nix to be run with the latest tools instead of
the pinned ones when desired, which is probably not very often,
but useful nonetheless.
nixpkgs-review is endorsed in CONTRIBUTING.md, as well as in the default
PR description template. It would be very useful to include this utility
in our shell.nix.
Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>
