Add nix-shell for working on nix-tools (input-output-hk#74)

Author: hamishmack

nix-tools/.buildkite/nix1/default.nix

@@ -1,3 +1,5 @@
+index-state: 2020-01-10T00:00:00Z
+
 packages: .
 
 -- Needs https://github.com/input-output-hk/iohk-nix/commit/6a8c29117eff36ce975e02e01efc8b25d93fcb90#diff-6fb0c6517b547a8baf082d5d2d604842
@@ -16,4 +18,3 @@ source-repository-package
     location: https://github.com/galenhuntington/haskell-src-meta.git
     tag: 109ee29d5fd0f4e23fdd2f80eb122d66341b64a9
     --sha256: 08qw6y9br6fy3qkwl9v2kp38msprsq9v1ssym0fsnj2jm0vbnfrx
-

nix-tools/.buildkite/nix2/default.nix

@@ -0,0 +1,13 @@
+{ haskellNixSrc ? builtins.fetchTarball {
+      url = "https://github.com/input-output-hk/haskell.nix/archive/a84e3b55c642e7aec57c9677158cf446d90e9759.tar.gz";
+      sha256 = "0qnchf0f8d5k6363mcrqv936wz6zljs06bjvg1i6xmkg31bdj48m";
+    }
+, nixpkgs ? haskellNixSrc + "/nixpkgs"
+, pkgs ? import nixpkgs (import haskellNixSrc)
+, haskellCompiler ? "ghc865"
+}:
+  pkgs.haskell-nix.cabalProject {
+    src = pkgs.haskell-nix.haskellLib.cleanGit { src = ./.; };
+    ghc = pkgs.haskell-nix.compiler.${haskellCompiler};
+  }
+

nix-tools/cabal.project

@@ -29,7 +29,7 @@ library
                      , Stack2nix.Project
                      , Stack2nix.Stack
   build-depends:       base >=4 && <4.13
-                     , Cabal >= 2.4
+                     , Cabal >= 2.4 && <3
                      , aeson
                      , aeson-pretty
                      , base16-bytestring

nix-tools/default.nix

@@ -0,0 +1,3 @@
+{ haskellCompiler ? "ghc865" }:
+(import ./. { inherit haskellCompiler; }).shells.ghc
+