2019-08-17

Upgrade to latest nixpkgs master.

From #6 (comment):

Problems I encountered and dealt with:

• krb5 / curl linking woes
  • fetchurl not being overridable: NixOS/nixpkgs#66503
  • fetchurl hard-enabling libkrb5 support for curl: NixOS/nixpkgs#66506
  • also fixes #46
• zlib confusion: NixOS/nixpkgs#66486
• inability to build both .a and .so for zlib: NixOS/nixpkgs#66490
• fontforge inexplicably failing when it seemed to have worked before and on cache.nixos.org (was a fixed-output bug): NixOS/nixpkgs#66598
• fixing fontforge by correcting old dumb mistake I had made in my python-fixing overlay: 1a62682
• Update stack2nix to handle stack2nix-output.nix: commitBuffer: invalid argument (invalid character) error like channable/vaultenv#43 (comment)
• Pin stack2nix to older version because it can't compile against Stack >= 2.0
• Ensure everything builds on CI: https://buildkite.com/nh2/static-haskell-nix/builds/66

Extra abilities unlocked:

• Remove need for zlib hardcode: 4521a44

We also now build stack vai stack2nix on CI for a big-sized test of the stack2nix integration.