We can't point it at a Nix expression that evaluates to our desired version of
nixpkgs because certain consumers want it to be a filesystem path instead.

This is what we wanted and much more directly.  It also avoids the gradual
lengthening of the path by always setting the path to nixpkgs-2105 instead of
some derivative of whatever the path was last time.

I hope I'm almost done with this branch.

Also add my key to local secrets for the moment because otherwise localdev
grid cannot build itself :/