Forbid runtime references to boost

We explicitly hack around to remove them, so might as well check that
the hack is useful.

(Introduced because I feared that the changes of
https://github.com/NixOS/nix/pull/5906#discussion_r784810238 would bring
back some runtime references)

flake.nix

@@ -299,6 +299,8 @@
 
           propagatedBuildInputs = propagatedDeps;
 
+          disallowedReferences = [ boost ];
+
           preConfigure =
             ''
               # Copy libboost_context so we don't get all of Boost in our closure.