Add simple test to check if nix is working

Author: avdv

rules_haskell_nix/BUILD.bazel

@@ -21,3 +21,10 @@ buildifier_test(
     ],
     verbose = True,
 )
+
+sh_test(
+    name = "nix_smoke_test",
+    size = "medium",
+    srcs = ["nix_test.sh"],
+    tags = ["external"],
+)

rules_haskell_nix/nix_test.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+export XDG_CACHE_HOME=${TMPDIR}
+
+exec nix run -I nixpkgs=../nixpkgs 'nixpkgs#cowsay' -- hello, rules_haskell_nix