tools/: add nixops

This now means we require Nix to be installed globally. This shouldn't
be the case in the long run, but will be until
https://github.com/tweag/rules_nixpkgs/issues/75 gets fixed or we maybe
move from rules_nixpkgs to nix-bundle or something similar.
diff --git a/WORKSPACE b/WORKSPACE
index 5ed4df0..c6bc0e1 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,6 +1,27 @@
 load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
+# Nix rules
+http_archive(
+    name = "io_tweag_rules_nixpkgs",
+    strip_prefix = "rules_nixpkgs-364adc70f32c2b3982995fce06b334f4d159070e",
+    urls = ["https://github.com/tweag/rules_nixpkgs/archive/364adc70f32c2b3982995fce06b334f4d159070e.tar.gz"],
+)
+
+# Nix packages
+
+load("@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", "nixpkgs_git_repository", "nixpkgs_package")
+nixpkgs_git_repository(
+    name = "nixpkgs",
+    revision = "2f1f9a9fe8a3c22f0677733523eaf6bd33995d50",
+)
+
+nixpkgs_package(
+    name = "nixops",
+	attribute_path = "nixops",
+    repositories = { "nixpkgs": "@nixpkgs" },
+)
+
 # Python rules
 git_repository(
     name = "io_bazel_rules_python",