release-cross.nix: fix cross bootstrap tools eval
release-cross.nix: fix cross bootstrap tools eval

In 5643714dea5, I changed pkgs/stdenv/linux/make-bootstrap-tools.nix
to take a package set instead of system and localSystem arguments, but
I forgot to update make-bootstrap-tools-cross.nix.

Fixes: 5643714dea5 ("stdenvBootstrapTools: inherit {cross,local}System")

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