go/workspace: implement EvalHscloudNix

This allows us to access hscloud nix 'facts' from Go.

Change-Id: Ic8fc3350a7d073947c44529fcae0bbb8627421aa
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1508
Reviewed-by: q3k <q3k@hackerspace.pl>
diff --git a/nix/readtree/BUILD b/nix/readtree/BUILD
new file mode 100644
index 0000000..4a80df2
--- /dev/null
+++ b/nix/readtree/BUILD
@@ -0,0 +1,2 @@
+# Used by //go/workspace tests.
+exports_files(["default.nix"])