Add a huge ugly hack for golden tests.

nix/shell.nix

@@ -0,0 +1,8 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  patched-yq = import ./yq.nix { inherit pkgs; };
+in
+pkgs.mkShell {
+  buildInputs = with pkgs; [ pkg-config patched-yq zlib ];
+}

nix/yq.nix

@@ -0,0 +1,24 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+# Make yq work inside whatever `stack test` launches.
+
+let
+  yq-patch = pkgs.writeText "yq-tty-fix.patch" ''
+diff --git a/yq/__init__.py b/yq/__init__.py
+index d95ac4b..ae7d59e 100644
+--- a/yq/__init__.py
++++ b/yq/__init__.py
+@@ -110,7 +110,7 @@ def cli(args=None, input_format="yaml", program_name="yq"):
+     in_place = args.in_place
+     delattr(args, "in_place")
+
+-    if sys.stdin.isatty() and not args.input_streams:
++    if (sys.stdin is None or sys.stdin.isatty()) and not args.input_streams:
+         parser.print_help()
+         sys.exit(2)
+     elif not args.input_streams:
+  '';
+in
+pkgs.yq.overrideAttrs (final: old: {
+    patches = old.patches ++ [ yq-patch ];
+})