Store gc-links more permanently

[mkg20001]

Currently we throw them in /tmp, where they get deleted, even though the update process state itself gets shoved into persistent state
Instead there should be some task-specific directory in gcroots, for ex /nix/var/nix/gcroots/per-user/_sunshine/$taskId that the task can self-clear on end (also scripts.js needs support for that aswell)