metamath: 0.172 -> 0.178
metamath: 0.172 -> 0.178

This also moves away from my mirror of the distributiont one hosted by
the metamath organization, which should generally be far more
up-to-date. However, that doesn't include any data files, so we need to
make sure we don't try to make those.

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