Tmpdir too small on isotesters

On branches that enable more of the fragile tests (in particular the USB upgrade feature), the runs always fail on scenarios “Try to "Upgrade from ISO" Tails to a pristine USB drive” and “Try to "Upgrade from ISO" Tails to a USB drive with GPT and a FAT partition” because /tmp/TailsToaster/ size is a bit too short. Error message is:

Error creating disk "pristine" in "/tmp/TailsToaster/TailsToasterStorage". Need 4096 MiB but only 4154 MiB is available of which 500 MiB is reserved for other temporary files.

So there actually seems to be enough free space, but some is reserved.

Related issues

Is duplicate of #11582 (closed)

Original created by @bertagaz on 11618 (Redmine)

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