mupdf: Patch out which
mupdf: Patch out which

This popped up while fixing up the expression for Darwin. Apparently
`which` isn't on the PATH there for Nix builds. May be considered an
impurity on Linux. `command -v` is the POSIX way to do basically the
same thing so I just patched out `which`.

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