Use TMPDIR when downloading state

[Hellzy]

This is a small QOL update to the bin/download_state.sh script.

Change

Use TMPDIR (default to /tmp if not set) to get rid of hardcoded /tmp path when invoking mktemp

Motivation

Some users may have a tmpfs located somewhere else, or too small to accommodate for the state file, in which case being able to relocate the download folder is handy to avoid jumping through hoops to update the node state.

Related issue: #201

[Neotamandua]

I suggest using an explicit --tmpdir DIR option instead of inheriting TMPDIR.

That solves:

1. TMPDIR being stripped by sudo, so the current change may silently fall back to /tmp.
2. This script runs as root, so accepting an arbitrary temp directory from the environment introduces a pathname-race risk if that directory or one of its parents is writable by an unprivileged user.

So having something like:
sudo download_state --tmpdir /mnt/dusk-tmp

& then validate that the resolved directory and its parents are root-owned (and reject group/world-writable directories unless they have protections like /tmp that prevent other users from replacing files).