unbound: don't run tests when cross-compiling

stdenv.mkDerivation will automatically set doCheck to false when
cross-compiling (which is why the default checkPhase doesn't happen).

pkgs/tools/networking/unbound/default.nix

···
       configureFlags="$configureFlags --with-nettle=${nettle.dev} --with-libunbound-only"
       configurePhase
       buildPhase
-      checkPhase
+      if [ -n "$doCheck" ]; then
+          checkPhase
+      fi
       installPhase
     ''
   # get rid of runtime dependencies on $dev outputs