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/tools/BUILD b/tools/BUILD
index d80ad4d..80678ca 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -55,3 +55,9 @@
     src = "@com_github_cloudflare_cfssl//cmd/cfssl:cfssl",
     visibility = ["//visibility:public"],
 )
+
+copy_go_binary(
+	name = "nixops",
+    src = "@nixops//:bin",
+    visibility = ["//visibility:public"],
+)